This is from the past real conditions of Farnell’s book:
εἴδει σ’ εἰδέναι τὰ ὑπὸ τούτων ῥηθέντα εἰ στρατηγὸς εἶναι ἔφησθα
The first part I basically get:
It was necessary for you to know the things spoken by these (people)…
the last bit, the prostasis has me completely stumped.
I’m not even sure whether ἔφησθα is the subjunctive of ἐφίημι or the imperfect of φημί and which ever I try I can’t work out what εἶναι is doing.
Any suggestions?
I assume εἴδει is a type for ἔδει
Two imperfects = unreal proposition.
“You would have known what they said, had you said you were a general.”
ἔφησθα is imperfect of φάναι. εἶναι refers back to the subject of the sentence, in this case, σύ.
εἴδει σ’ εἰδέναι τὰ ὑπὸ τούτων ῥηθέντα εἰ στρατηγὸς εἶναι ἔφησθα
εἴδει verb 2nd sg pres ind mid of εἴδομαι, to appear or seem to be + inf. case
ἔφησθα verb 2nd sg imperf ind act of φημί, to say, affirm
This is a simple condition in reverse.
“You appear to know the things that had been said by these men if you said that you were a general.”
Now it seems to make sense - thanks.
It was necessary for you to know the things spoken by these (people) if you were claiming to be general.
Yup, a typo 
But surely it would have αν if it were unreal. On top of that the exercise was flagged up as real past conditions (that is to say, open) and Farnell has not yet covered past unreal conditions.
Even though ἔδει was in fact a typo I am glad to see that εἴδει makes sense. Your wording also helps clarify the meaning of the actual wording so you certainly have my thanks for that as well.
ἔδει σ’ εἰδέναι … “You should have known …”, “You had to know …”. This is the main clause (“apodosis” if you must). The if-clause makes no difference to it.
(Incidentally, try to get out of the habit of translating δει as “it is necessary.” It doesn’t mean that.)
εἰ στρατηγὸς εἶναι ἔφησθα. “If you said you were the strategos”, “If you were claiming to be the strategos.”
φημι takes (acc.&)inf., indirect statement. Here there’s no acc. because the subject of ειναι is the same as the subject of εφησθα, i.e. “you.” Hence στρατηγος, the predicate, is nominative. Cf. στρατηγος ειμι (I’m a strat.) and στρατηγος ειναι φημι (I say I’m a strat., I claim to be a strat.).
Contrast στρατηγὸν εἶναι φημί (acc.&inf.), I say there is a strategos.
If it helps, you can refer to my post on conditionals, which avoids the need for all the various labelling systems, http://discourse.textkit.com/t/conditionals-a-guide/14120/1
ειδει would not in fact make sense, followed as it is by σ(ε).