1. μή is the negative of a direct expression of a wish, and of all indirect expressions of wish except πῶς ἄν with the optative and a form of βούλομαι with the infinitive.
2. Wishes for the future, whether the object of the wish is reasonable or unreasonable, attainable or unattainable, are expressed by the optative with or without εἴθε or εἰ γάρ ( cross1814, cross1815). Indirect expressions are: πῶς ἄν with the optative ( cross1832); βουλοίμην ἄν with the infinitive ( cross1827).
3. Wishes for the present: that something might be otherwise than it now is, are expressed by the imperfect with εἴθε or εἰ γάρ ( cross1780). Indirect expressions are: ὤφελον (with or without εἴθε or εἰ γάρ) and the present or aorist infinitive ( cross1781); ἐβουλόμην (with or without ἄν) with the infinitive ( cross1782, cross1789).
4. Wishes for the past: that something might have been otherwise than it then was, are expressed by the aorist indicative with εἴθε or εἰ γάρ ( cross1780). Indirect: ὤφελον (with or without εἴθε or εἰ γάρ) with the present or aorist infinitive ( cross1781).
5. Unattainable wishes for the present or past may be entirely reasonable.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].