Herbert Weir Smyth, A Greek Grammar for Colleges

a. In the narration of past events, the subjunctive sets forth a person's previous purpose in the form in which he conceived his purpose. Thus (τὰ πλοῖα) Ἀβροκόμα_ς . . . κατέκαυσεν ἵνα μὴ Κῦρος διαβῇ Abrocomas burned the boats in order that Cyrus might (may) not cross X. A. 1.4.18. Here the thought of A. was ‘I will burn the boats that Cyrus may not cross’ (ἵνα μὴ διαβῇ), and is given in a kind of quotation.

N.—Thucydides and Herodotus prefer this vivid subjunctive; the poets, Plato, and Xenophon, the optative. In Demosthenes, the subjunctive and optative are equally common.

b. When the purpose (or its effect) is represented as still continuing in the present. See the example in 2195. This use is closely connected with a.

c. After τί οὐ, τί οὖν οὐ, and the aorist indicative: τί οὖν οὐχὶ τὰ μὲν τείχη φυλακῇ ἐχυρὰ ἐποιήσαμεν ὅπως ἄν (2201) σοι σᾶ ᾖ κτλ.; why then do we not make your walls strong by a garrison that they may be safe for you, etc.? X. C. 5.4.37. Here the sentence with ἐποιήσαμεν is practically equivalent to one with ποιήσωμεν.