Languages often have multiple means of communicating the same thing. Lexical inventories overlap; grammatical forms might share related functions. Today, we are exploring an example that in a sense straddles both, though it certainly leans toward the lexical side, especially at first glance. Δύναμαι is a middle-only (media tantum) verb that functions to express that the subject has an ability to perform some action: they are able to do something. It is most commonly used with an infinitive:

1. δύνασθε πιεῖν τὸ ποτήριον ὃ ἐγὼ πίνω, ἢ τὸ βάπτισμα ὃ ἐγὼ βαπτίζομαι βαπτισθῆναι; Are you able to drink the cup that I drink or be baptized with the baptism that I am baptized? (Mark 10:38)
2. Ἀπολελύσθαι ἐδύνατο ὁ ἄνθρωπος οὗτος εἰ μὴ ἐπεκέκλητο Καίσαραa
   This man could have been released if he had not appealed to Caesar (Acts 26:32).
3. οὐ δύναται δὲ ὁ ὀφθαλμὸς εἰπεῖν τῇ χειρί· Χρείαν σου οὐκ ἔχω
   The eye cannot say to the hand, I have no need of you (1 Corinthians 12:21).

But there is another verb that has a comparable meaning that may also take an infinitive: ἰσχύω, ‘I am healthy, strong, am able’, which is also, incidentally, an active-only (activa tantum) verb. When it takes an infinitive it apparently functions in more or less the same way. The question is whether or not these two words when used in their infinitival constructions may be distinguished in their usage.

One thing that is interesting about both verbs is that they have a very strong propensity for usage in negated clauses: it is far more common for someone to assert a lack of ability than the reverse:

4. οὐκέτι αὐτὸ ἑλκύσαι ἴσχυον ἀπὸ τοῦ πλήθους τῶν ἰχθύων
   They were not able to haul it out because of the number of fish (John 21:6).
5. οὐδεὶς ἴσχυεν αὐτὸν δαμάσαι
   Nobody was able to subdue him [a demon possessed man] (Mark 5:4).
6. οὐκ ἠδυνήθησαν εἰσελθεῖν διʼ ἀπιστίαν
   They were not able to enter because of their unbelief (Hebrews 3:19).
7. οὐ δύνασθε ποτήριον κυρίου πίνειν καὶ ποτήριον δαιμονίων
   You cannot drink the cup of the Lord and the cup of demons (1 Corinthians 10:21).

The difference between the two seems to be one of semantic range. While both verbs denote ability, ἰσχύω has a much narrower span of usage. It is primarily limited to physical ability.

8. μὴ ἰσχύειν τινὰ παρελθεῖν διὰ τῆς ὁδοῦ ἐκείνης
   No one was able to pass by on the road.
9. Οὕτως οὐκ ἰσχύσατε μίαν ὥραν γρηγορῆσαι μετʼ ἐμοῦ;
   You couldn’t stay awake even just one hour with me?

But δύναμαι has lexicalized a wider range of modal expression.

9. Μὴ δύνανται οἱ υἱοὶ τοῦ νυμφῶνος ἐν ᾧ ὁ νυμφίος μετʼ αὐτῶν ἐστιν νηστεύειν;
   How can the guests of the bridegroom fast while the bridegroom is with him? (Mark 2:19).

Here in Mark 2:19, the usage of δύναμαι does not signal a lack of ability to fast, but that such behavior is socially inappropriate and absurd.

Similarly, in Mark 6:5, Jesus has not lost his capacity to do miracles, but he intentionally chooses not to do any because he knows that their effect would be wholly lost on the audience:

10. καὶ οὐκ ἐδύνατο ἐκεῖ ποιῆσαι οὐδεμίαν δύναμιν, εἰ μὴ ὀλίγοις ἀρρώστοις ἐπιθεὶς τὰς χεῖρας ἐθεράπευσεν
    And he was not able to do any miracles there, except laying his hands on a few sick people, he did some healing (Mark 6:5).

Mark 6:19 extends the usage of δύναμαι from simple ability into the domain of permission/prohibition:

11. ἡ δὲ Ἡρῳδιὰς ἐνεῖχεν αὐτῷ καὶ ἤθελεν αὐτὸν ἀποκτεῖναι, καὶ οὐκ ἠδύνατο
    Herodias held a grudge against him and wanted to kill him and was not able [= was not allowed]  (Mark 6:19).
12. μή τις δύνηται ἀγοράσαι ἢ πωλῆσαι εἰ μὴ ὁ ἔχων τὸ χάραγμα, τὸ ὄνομα τοῦ θηρίου ἢ τὸν ἀριθμὸν τοῦ ὀνόματος αὐτοῦ
    No one was allowed to buy or sell except those who had the mark, the name of the beast or the number of his name (Rev 13:17).

In John 1:46, Nathanael uses δύναμαι as an epistemic modal expressing the lack of possibility rather than ability.

13. Ἐκ Ναζαρὲτ δύναταί τι ἀγαθὸν εἶναι;
    Can anything good come from Nazareth? (John 1:46).

Indeed, in rhetorical questions, δύναμαι is often used to express a low degree of probability rather than absolute ability, physical or otherwise.

14. Σκληρός ἐστιν ὁ λόγος οὗτος· τίς δύναται αὐτοῦ ἀκούειν;
    This saying is hard. Who can understand it? (John 6:60)

In normal (non-rhetorical) questions, δύναμαι can be used to inquire about the probability/possibility of an event.

15. Δυνάμεθα γνῶναι τίς ἡ καινὴ αὕτη ἡ ὑπὸ σοῦ λαλουμένη διδαχή;
    Can we learn this new thing that is proclaimed by you? (Acts 17:19).

The majority of examples of ἰσχύω+infinitive are limited to the domain of physical capacity:

16. πλημμύρης δὲ γενομένης προσέρηξεν ὁ ποταμὸς τῇ οἰκίᾳ ἐκείνῃ, καὶ οὐκ ἴσχυσεν σαλεῦσαι αὐτὴν διὰ τὸ καλῶς οἰκοδομῆσθαι αὐτήν
    When a flood came, the river burst against that house and it was not able to shake it because it was so well built (Luke 6:48).
17. προσαναλώσασα ὅλον τὸν βίον οὐκ ἴσχυσεν ἀπʼ οὐδενὸς θεραπευθῆναι
    Though she spent her entire livelihood, she could not be healed by anyone (Luke 8:43).
18. Οὗτος ὁ ἄνθρωπος ἤρξατο οἰκοδομεῖν καὶ οὐκ ἴσχυσεν ἐκτελέσαι
    This man began to build and was not able to finish it (Luke 14:30).

The closest we get to finding examples of ἰσχύω+infinitive being extended beyond the domain of physical ability involves examples where the Pharisees interact with Jesus. If an action is attempted, but thwarted, ἰσχύω+infinitive can be used:

19. καὶ οὐκ ἴσχυσαν ἐπιλαβέσθαι τοῦ ῥήματος ἐναντίον τοῦ λαοῦ
    And they were not able to catch him in his word in front of the people (Luke 20:26).

Similarly, at another moment when the Pharisees are attempting to trap Jesus, his response is so effective that they are silenced. Here and above, Luke’s use of ἰσχύω+infinitive makes for a stronger assertion about their lack of ability than would have been expressed with δύναμαι.

20. καὶ οὐκ ἴσχυσαν ἀνταποκριθῆναι πρὸς ταῦτα
    And they were not able to reply to this (Luke 14:6).

By mapping the narrower physical semantic range of ἰσχύω onto contexts of riposte and social conflict, Luke is effectively saying that they were rendered mute by Jesus’ response. Luke describes Stephen’s speaking to have a similar effect in Acts 6:10:

21. καὶ οὐκ ἴσχυον ἀντιστῆναι τῇ σοφίᾳ καὶ τῷ πνεύματι ᾧ ἐλάλει.
    And they were not able to oppose the wisdom and the spirit of his speech (Acts 6:10).

Here again the skill of debating is construed by Luke as a physical ability that when challenged by someone else with a greater ability cannot stand. This is a metaphoric extension that activates the conceptual metaphor ARGUMENTS ARE WAR (Lakoff and Johnson 1980) that maps rhetorical skills onto physical strength and ability (see also Acts 25:7 during Paul’s trial).

One final comment: on the whole, while ἰσχύω+infinitive is used by Matthew, Mark, John, and Luke, it is never used by Paul or any other author in the New Testament. This is a predominantly narrative usage, which by itself is striking. This is not something I am confident I can explain without more investigation. But it is an obvious fact. Both usages in Matthew and Mark parallel each other to some extent: Matt 8:28 and Mark 5:4, albeit not remotely identical, both involve Jesus’ encounter with demon possession at the Gerasenes. Matt 26:40 and Mark 14:37 are clear synoptic parallels with identical word order and only a shift in grammatical number (2SG vs. 2PL) and Matthew’s addition of μετʼ ἐμοῦ at the end. This reduces the true number of non-Lukan examples.

Additionally, it is only in Luke’s writings where the usage is extended to non-physical domains, something that does happen elsewhere, but tends to be a sign of better writing style: creative language use and the introduction of more and more complex metaphorical construals is one sign of writing skill. This bears itself out in other narrative texts, particularly 1-4 Maccabees. One would imagine that if the author of Hebrews wrote some narrative, we would find the same.