Herbert Weir Smyth, A Greek Grammar for Colleges

1. Statements of Fact (direct assertions) as to the present, past, or future are made in the indicative mood (negative οὐ), 1770.

A. Statements of fact include statements of present, past, or future possibility, likelihood, or necessity, which are expressed by the indicative of a verb denoting possibility, likelihood, or necessity, and an infinitive (1774-1779).

B. Statements of customary or repeated past action are made in the imperfect or aorist indicative with ἄν (negative οὐ), 1790.

2. Statement of Opinion (usually cautious, doubtful, or modest assertions) as to what may be (might be), can be (could be), may (might, could, would) have been, etc., are made:

A. In reference to the present or past: by ἐβουλόμην ἄν I should like or I should have liked (negative οὐ), 1789. (Rarely by the indicative without ἄν, negative μή or μὴ οὐ, 1772.)

B. In reference to the past: by the aorist or imperfect indicative with ἄν (negative οὐ), 1784, cp. 1786.

C. In reference to the present (statement of present opinion the verification of which is left to the future): by the optative with ἄν (negative οὐ), 1824.

D. In reference to the future: by the present subjunctive with μή or μὴ οὐ (1801); by οὐ μή with the aorist subjunctive to denote an emphatic denial (1804).