Herbert Weir Smyth, A Greek Grammar for Colleges

a. The compound force of μὲν οὖν is seen in affirmations; as in replies: πάνυ (μάλιστα) μὲν οὖν yes, by all means; certainly, by all means; aye truly, εὖ μὲν οὖν οἶδα nay, I am sure of it, οὐ μὲν οὖν indeed not, ““ἆρ᾽ ου᾽ τόδε ἦν τὸ δένδρον ἐφ᾽ ὅπερ ἦγες ἡμᾶς; τοῦτο μὲν οὖν αὐτό” isn't this the tree to which you were bringing us? To be sure this is it” P. Phae. 230a.

b. The compound force appears also when μὲν οὖν indicates a correction; nay rather (imo vero); as λέγε σύ: σὺ μὲν οὖν μοι λέγε do you say. Nay, rather you Ar. Eq. 13, ἄτοπον τὸ ἐνύπνιον, ὦ Σώκρατες. ἐναργὲς μὲν οὖν the dream is strange, Socrates. Nay rather, it was distinct P. Cr. 44b.

c. Each particle has its own force especially where μὲν οὖν indicates a transition to a new subject. Here μέν points forward to an antithesis to follow and indicated by δέ, ἀλλά, μέντοι, while οὖν (inferential) connects with what precedes. Here so then, therefore may be used in translation. Thus, ““Κλέαρχος μὲν οὖν τοσαῦτα εἶπε. Τισσαφέρνης δὲ ὧδε ἀπημείφθη” such then were the words of Clearthus; and on the other hand Tissaphernes answered as follows” X. A. 2.5.15 Sometimes μὲν οὖν (like igitur) shows that a subject announced in general terms is now to be treated in detail (P. Ph. 70c).