μὴ θαυμάσητε δὲ τὸ μῆκος τῆς ἐπιστολῆς
Do not be surprised at the length of the letter.
The aorist subjunctive, preceded by μή, signals a prohibition, as the negation of an aorist imperative.
This prohibition is usually in the second person, very rarely in the third. For the third person μή with the aorist imperative is more common.
The use of μὴ aorist subj. (at the expense of the parallel construction μὴ present imp.) occurs mainly in:
- specific prohibitions
- polite prohibitions
- prohibitions involving telic verbs
In recent Ionic and in Attic ὅπως μή is occasionally used instead of μή (sometimes accompanied by a future indicative instead of an aorist subjunctive).
The use of the subjunctive for a prohibition could be explained - following Rijksbaron - on the basis of a nuance of fear.
εἰ μὴ καθήκει, μὴ πράξῃς· εἰ μὴ ἀληθές ἐστι, μὴ εἴπῃς
If it is not good, do not do it; if it is not true, do not say it.
μὴ δῷς τὰ σεωυτοῦ ἀγαθὰ ἄλλοισι.
Do not give your own good qualities to others.
καὶ μηδεὶς ὑπολάβῃ με δυσκόλως ἔχει
And nobody should think that I am hard to please.