Since this post is appearing far later than I had intended, I would encourage you to quickly go back and review the previous post in the series: Linguistic adequacy and Greek grammar.
I must apologize first for the fact that we’re starting with BDF (Amazon Link). It is an incredibly dense and difficult to read volume. I’ve heard it said that if you know Greek well enough to have the ability to use BDF, then you probably don’t even actually need to use BDF anyway. And perhaps there’s some truth to that. Before moving forward with our examination of this grammar, however, we should talk a bit about the context in which it was written. Grammar writing in and of itself is a communicative act and grammars are written for particular audiences who expect to use them for particular purposes. This is true even of reference grammars (or perhaps especially of reference grammars). A reference grammar written for native speakers of a language has different goals than a reference grammar written for non-native speakers, for linguists/grammarians, or, in the case of most biblical students and scholars, those who are centrally interested in reading. Moreover, there is a tendency for each of these groups to assume that the wants and needs of the other groups ought to be identical to their own. These groups are interested in different questions when they pull the grammar off the bookshelf. BDF seems to fit primarily within the final two groups, perhaps with some greater emphasis on the last one. It’s also notable in that the grammar functions primarily as a supplement to Classical Greek—much of the discussion is framed in terms of how the Koine diverges from the classical standard.
I chose the following section from BDF primarily because it was something I read recently. It is probably worth emphasizing here that I did not decide to write about linguistic adequacy because of something I read in this section. I had the thought while actually reading a different grammar in conjunction with a conversation I had with a friend. I decided, then, that I would examine and evaluate some of the more prominent reference grammars to see how they stand up to contemporary linguistic standards of adequacy.
The section before us from BDF below comes from a larger section that examines (semantic) types of subordinate clauses in relationship to the indicative and subjunctive moods.
369. Final (purpose) clauses introduced by ἵνα, ὅπως (no longer with ὡς except in A 20:24 S* [§391(1)]), μή have greatly extended their sphere in the NT because a ἵνα-clause so often serves as periphrasis for the infinitive. We are concerned here with mood only, upon which the character of ἵνα (i.e. whether it indicates purpose or not) exerted no influence. (1) The mood in the NT is generally the subjunctive. The classical ‘oblique optative’ is never used even after a secondary tense in the NT nor elsewhere in the lower Koine vernacular; cf. Knuenz 15ff. (2) The future indicative has also been introduced to a very limited degree in the very places where it would not have been permissible in classical, i.e. after ἵνα and final μή, most frequently in Rev and usually with the aorist subjunctive as variant. (3) A special case is that in which a future connected by καί follows upon ἵνα or μή with the subjunctive to designate some further consequence: Jn 15:8 ἵνα καρπὸν … φέρητε καὶ γενήσεσθε (γένησθε BDL al.) ἐμοὶ μαθηταί, where the consequence has a kind of independence: ‘and then you will become …’. It is still more easily understood when it follows an independent subjunctive: Mk 6:37 ἀγοράσωμεν καὶ δώσομεν (p45ALΔ, -σωμεν SBD, al. δῶμεν). (4) The old Attic (Meisterhans 255) combination of ὅπως and ὅπως μή with the future indicative after verbs of reflection, striving, guarding is not found in the NT. Ἵνα (negated ἵνα μή, μή) is used throughout with these verbs, and ὅπως, in so far as it appears at all (never in Rev, once in Jn, not often in Paul), is confined to the purely final sense and to combinations with verbs of asking (παρακαλεῖν etc.). (5) Furthermore, ὅπως, with the exception of a few places in Lk and a quotation from the LXX, has lost the ἄν often appended in Attic (Hermann 267f.; Knuenz 13ff., 26ff.; Rosenkranz, IF 48  166), especially in the older inscriptions (Meisterhans 254). Ἄν could not be joined to ἵνα and μή even in Attic. (6) The present indicative after ἵνα is, of course, only a corruption of the text.—For μή (μήποτε, μήπως) expressing apprehension s.
The basic structure of BDF is to make a set of numbered statements about grammatical aspects of the language. These statements are then followed by corresponding set of numbers lists of references. These references with the briefest of commentary are intended to function as the evidence or examples for the corresponding grammatical claims. Often times, debated examples are provided with some comment as to how they should be interpreted. BDF is known for its use of actual manuscripts rather than critical editions. This substantively contributes to the value since it allows us some observation not only about the grammar of the text, but also some insight into the kinds of grammatical variation found in the manuscripts.
The main problem with the initial set of grammatical observations is essentially that they are very little than that: observations. Here they are in a list format, with my own comments about what I view to be lacking, primarily framed in terms of questions that remain unanswered from the statements themselves:
- (1) The mood in the NT is generally the subjunctive. The classical ‘oblique optative’ is never used even after a secondary tense in the NT nor elsewhere in the lower Koine vernacular; cf. Knuenz 15ff.
These statements are certainly true, but we are left wondering what motivated the disappearance of the ‘oblique optative.’ Does the dominance of the subjunctive in the NT and surrounding literature find it’s motivation in the language simplification processes that take place via koineization? Or should we be looking for some form of analogical leveling? Perhaps both? Does Knuenz 15ff. provide answers to these questions or does he simply contribute more data for this unmotivated observation?
- (2) The future indicative has also been introduced to a very limited degree in the very places where it would not have been permissible in classical, i.e. after ἵνα and final μή, most frequently in Rev and usually with the aorist subjunctive as variant.
What motivates this introduction of the future indicative? Does it involve some particular aspect of final purpose clauses? Or it is primarily motivated by the nature of the future indicative as a semi-modal form to begin with? Is it limited to a specific semantic class of verbs (as with  below)? Surely such a change from Classical Greek to the the New Testament suggests at least some kind of semantic shift in order to motivate this new usage.
Also , when you say that the aorist subjunctive is usually a variant, why do you think that is? Is it primarily because of the morphological similarities between the aorist subjunctive and the future indicative? Can we attribute some semantic significance to the alternation of these variants?
- (3) A special case is that in which a future connected by καί follows upon ἵνα or μή with the subjunctive to designate some further consequence: Jn 15:8 ἵνα καρπὸν … φέρητε καὶ γενήσεσθε (γένησθε BDL al.) ἐμοὶ μαθηταί, where the consequence has a kind of independence: ‘and then you will become …’. It is still more easily understood when it follows an independent subjunctive: Mk 6:37 ἀγοράσωμεν καὶ δώσομεν (p45ALΔ, -σωμεν SBD, al. δῶμεν).
This is probably the best statement we have so far. Here we have a data point: future connect by καἰ follows upon ἵνα or μή with the subjunctive” and then also a statement of the meaning of the construction. Unfortunately, what motivates the construction still is unstated. Why does this happen rather than merely add another subjunctive (which is what happens in BDF’s variant reading, which also happens to be the reading of the NA27)?
- (4) The old Attic (Meisterhans 255) combination of ὅπως and ὅπως μή with the future indicative after verbs of reflection, striving, guarding is not found in the NT. Ἵνα (negated ἵνα μή, μή) is used throughout with these verbs, and ὅπως, in so far as it appears at all (never in Rev, once in Jn, not often in Paul), is confined to the purely final sense and to combinations with verbs of asking (παρακαλεῖν etc.).
Like statement (3), this is also more useful, BDF against connect structure and meaning together. We have an old construction with its meaning (and lexical condition) followed by the new Koine conjunction and its meaning. We are still left with the question of motivation that keeps re-occurring with each of these.
- (5) Furthermore, ὅπως, with the exception of a few places in Lk and a quotation from the LXX, has lost the ἄν often appended in Attic (Hermann 267f.; Knuenz 13ff., 26ff.; Rosenkranz, IF 48  166), especially in the older inscriptions (Meisterhans 254). Ἄν could not be joined to ἵνα and μή even in Attic.
Okay. But what is it about the semantic nature of ἄν that prevents its collocation with ἵνα and μή? The fact that it doesn’t happen is certainly interesting, but surely the reason why would be even more interesting.
- (6) The present indicative after ἵνα is, of course, only a corruption of the text.—For μή (μήποτε, μήπως) expressing apprehension s. §370.
This is useful, however, could there not also be an alternative explanation that an author simply made a production error? The use of μή to express apprehension, quite interesting. That would come pretty close to functioning as a communicative motivation for a grammatical structure.
But what we see as a whole is very plenty of observational statements of fact with no explanation of them. There is no principled account of why the data is the way it is. So the question then is: Is this observational account useful. Well, yes. It is to an extent. To the extent that the factoids collected here are accurate, they could function as an essential foundation for creating a principled account of why the data is the way it is. The fact of the matter is that BDF feels like the beginning of a grammar. All the data is collected and initial observations made, but there are no connections between the dots at all. Just data points floating around in space unrelated and without purpose.
Section §370. which follows at the end of this point for reference, functions as providing the specific examples that BDF views as relevant as evidence of their statements. This approach is as perplexing as it is interesting. In some sense, this becomes useful to commentary writers and those doing exegesis/general reading of Greek. The problem is that the way BDF is used and cited in commentaries isn’t useful at all. Consider Revelation 22:14
Μακάριοι οἱ πλύνοντες τὰς στολὰς αὐτῶν, ἵνα ἔσται ἡ ἐξουσία αὐτῶν ἐπὶ τὸ ξύλον τῆς ζωῆς καὶ τοῖς πυλῶσιν εἰσέλθωσιν εἰς τὴν πόλιν.
ἵνα can be construed several ways (see Notes 14:13.f. and 16:15.c.): (1) as introducing a final or purpose clause, i.e., “in order that they will have access to the tree of life” (BDF §369; Turner, Syntax, 102), (2) as introducing a causal clause, “because they will have access to the tree of life” (i.e., ἵνα = ὅτι, as the ἵνα in Mark 4:12 = Luke 8:10 becomes ὅτι in Matt 13:13), a usage that is also found in 14:13 and 16:15, or (3) as the imper use of ἵνα: e.g., “may their right be to the tree of life”.
Aune, Revelation 1198.
It is interesting here that only Aune’s option (1) actually reference citation support. But what I wonder is just how helpful the citation is. All BDF §369(2) says is that the future indicative with ἵνα is used to mark final purpose clauses. You cannot use BDF §369(2) to make a judgment about a particular instance because BDF makes no statement about why a particular construction that can be used to mark that meaning as opposed to another meaning. Once again we are back to the problem of having no principled account.
With BDF, the question never answered is: Why?
Will another, perhaps less terse, traditional grammars do better? We’ll see in the coming weeks.
Examples of Final purpose clauses from BDF
Pap. ὡς (ὡς ἄν) iii BC 4 times, ii–i BC 18 times; Mayser II 1, 258–61. Cf. Rob. 982. D. Buzy, Les sentences finales des paraboles évangéliques (RB 40  321–44). On ὅπως- and ἵνα-clauses in the Apocr. Gospels Ghedini, Vang. ap. 474–8. Kalinka, WSt 55 (1937) 91–4 (compendium of the results of the work of Knuenz).
(1) The alleged opt. δῴη E 1:17 is subj. (§95(2); B correctly δῷ). Τίς δώῃ = מִי יִתֵּן is frequent in the LXX and is an unmistakable subj. The subj. with ὅπως is aor. except ὅπως ᾖ Mt 6:4 and ὅπως μὴ δύνωνται Lk 16:26. Mayser II 1, 240ff. (subj.), 295 (opt.).
(2) Fut.: Rev 22:14 ἵνα ἔσται … καὶ εἰσέλθωσιν (ἵνα here = ‘because’ ὅτι, as in 14:13), therefore both forms thought of as equivalent (? s. infra). In Paul: 1 C 9:15 ἵνα τις (οὐδείς is incorrect) κενώσει, 18 ἵνα θήσω, 13:3 παραδῶ ἵνα καυθήσομαι (incorrect -σωμαι CK, καυχήσωμαι p46SAB), G 2:4 καταδουλώσουσιν (SAB*CDE), Ph 2:11 ACD al. Further: 1 P 3:1 κερδηθήσονται; Jn 7:3 θεωρήσουσιν (-σωσι B3X al.), 17:2 δώσει (-ῃ ScACG al., δώσω S*, δῷς W, ἔχῃ D); Lk 14:10 ἐρεῖ (ADW al. εἴπῃ), 20:10 δώσουσιν (CDW al. δῶσιν); Mt 12:10 κατηγορήσουσιν DWX. After μή: C 2:8 βλέπετε μὴ … ἔσται, H 3:12 βλέπετε μήποτε ἔσται, Mt 7:6 μήποτε καταπατήσουσιν (-σωσιν SEG al.) … καὶ ῥήξωσιν. Cf. also Gregory 124. Rev 3:9 ἵνα ἥξουσιν (-ωσι 046) καὶ προσκυνήσουσιν (-σωσιν 046) … καὶ γνῶσιν (S γνώσῃ is not good); 8:3 δώσει (-ῃ P 046), 13:16 (written ΔΩΣΙ from which the wrong reading δωσι(ν) in all majuscules arose); 6:4, 11, 9:4, 5, 20, 13:12, 14:13 (ὅτι p47). 1 Th 5:10 ἵνα ζήσομεν (A; ζῶμεν D*E; the aor. is correct S etc. ‘live again’, i.e. at the parousia); ἄν is also omitted in the intervening clause εἴτε γρηγορῶμεν εἴτε καθεύδωμεν (cf. Ph 1:27 ἵνα εἴτε … ἀκού(σ)ω [subj.], Homil Clem 9:22 ἵνʼ ….ὅτε θελήσωσιν, ἀνέλωσιν). Interchange of fut. indic. and aor. subj. (cf. §363): Reinhold 106; Raderm.2 173f.; Melcher 90; Vogeser 34f.; Knuenz 23ff., 39.—On ‘causal’ ἵνα (Rev 22:14) s. Jannaris §1741; Hesseling and Pernot, Neophilologus 12 (1927) 41–6; Pernot, Études 90–5; Windisch, ZNW 26 (1927) 203–9; Robertson, Studies in Early Christianity, ed. by S. J. Case (N.Y. and London, 1928) 51–7. Of the NT exx. adduced by Hesseling and Pernot, Rev 22:14 at the best stands the test if μακάριοι … ἵνα = μακάριοι … ὅτι of Mt 5:3ff.; but ‘in order that’ (dependent on πλύοντες) is also possible; likewise Rev 16:15. 14:13 ἵνα (p47 ὅτι!) ἀναπαήσονται is a main clause ‘they shall rest’ (§387(3)). Mk 4:12 = Lk 8:10 ἵνα is final (theory that some arire incapable of repentance), softened by Mt 13:13 to causal ὅτι (διὰ τοῦτο in answer to διὰ τί 10). Ed. Schweizer, ThZ 8 (1952) 153f. accepts ἵνα in 1 P 4:6 as causal. Literature on causal ἵνα also in Zerwick, Graec. bibl. 95 n. The LXX is also ruled out: Gen 22:14 ἵνα ‘so that’ (§391(5)), likewise Epict.: 3.4.10 ἵνα is final, cf. θέλω in 11. But still there remain the grammarians (e.g. Apollonius Dysc., Synt. 3.28 [Gramm. Gr. ii 2, 382.2] explains ἵνα φιλολογήσω παρεγενήθη Τρύφων as identical with διότι ἐφιλολόγησα π. Τ.), the Church Fathers and late papyri (e.g. BGU IV 1081.3 [ii/iii AD] ἐχάρην ἵνα σε ἀσπάζομαι [however cf. §392(1a)]; Ghedini, Aegyptus 15  236).
(3) A 21:24 ἵνα ξυρήσωνται (-σονται SB*D*E al.) … καὶ γνώσονται, E 6:3 OT ἵνα … γένηται καὶ ἔσῃ, R 3:4 OT ὅπως ἂν δικαιωθῇς … καὶ νικήσεις (SADE), Lk 22:30 (many vv.ll.), 12:58 (μήποτε), Mt 5:25 (likewise), Mk 5:23 (acc. to A), Mt 13:5 = Jn 12:40 = A 28:27 (μήποτε or ἵνα μή) OT (Is 6:10); Mt 20:28 add. D μήποτε … ἐπέλθῃ … καὶ καταισχυνθήσῃ; Barn 4.3 ἵνα ταχύνῃ καὶ ἥξει (S for -ῃ), Herm Man 6.2.10, Sim 9.7.6, 28.5; Raderm.2 216. Following an impera.: Herm Vis 2.1.3 λάβε καὶ ἀποδώσεις μοι, Man 2.1 ἄκακος γίνου καὶ ἔσῃ (Lat. esto) ὡς … (Lk 22:10 ἀκολουθήσατε … καὶ ἐρεῖτε); Raderm.2 216f.
(4) The one instance in Jn is 11:57 where ὅπως is evidently used for the sake of variety since ἵνα has just preceded; the same thing applies to Paul in 1 C 1:29, 2 C 8:14, 2 Th 1:12 (not 2 C 8:11, G 1:4, Phm 6; ἵνα … ἵνα G 4:5, 1 C 4:6). Further Epict. 4.5.5, Tatian 41.8 Schwartz. For the expression of purpose Jn uses hardly anything but ἵνα and does not care for the final inf. at all. On the retreat of ὅπως in the Hell. period cf. Reinhold 106; Knuenz 9ff., 28, 34ff.; Schwyzer II 673. However, in the Ptol. pap. ὅπως is almost as frequent as ἵνα but four-fifths of the exx. are in official documents (Mayser II 1, 247–52, 256, 257, 261). Ἵνα in final object clauses: ibid. 242ff.; II 3, 51. Ὅπως and ἵνα often interchange (ibid. II 1, 245; cf. Diog. Oen. 60.1.8 οὐχ ἵνα … ἀλλʼ ὅπως, 1 Clem 65.1 ὅπως … εἰς τό with inf.). The fut. also appears infrequently with ὅπως in the pap. (ibid. 251).
(5) Ὅπως ἄν: Lk 2:35, A 3:20, 15:17 OT (from Amos 9:12, where our text does not have ἄν [Ziegler except in A as a back-reading]); in quotation also R 3:4 = Ps 50 (51): 6 (R 9:17 OT acc. p 188 to FG; from Ex 9:16, where our text is without ἄν). Mayser II 1, 254–7 (ὅπως ἄν very often in the pap., predominantly in official texts); II 3, 50. An uncertain ex. of ἵνα ἄν may be found in Mayser II 1, 246.
(6) Jn 5:20 SL, G 6:12 p46ACF al., T 2:4 S*AF al., E 1:18 FG ἵνα οἴδατε, etc. Cf. Gregory 125; Reinhold 106. Φυσιοῦσθε 1 C 4:6 and ζηλοῦτε G 4:17 are subjunctives, s. §91. Jn 17:24 θεωροῦσιν W, -ῶσιν W2, Rev 16:15 βλέπουσιν p47 minusc. IEph 4.2 ἵνα … ᾄδετε and ἵνα … μετέχετε (Reinhold 107). Only inferior orthography in the Ptol. pap. (Mayser II 1, 244d 1).—On the whole Rob. 980–7; Moule 138f.