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://www.textkit.com/greek-latin-foru ... =2&t=64823
ειδει would not in fact make sense, followed as it is by σ(ε).