Herbert Weir Smyth, A Greek Grammar for Colleges

a. Also by poetic οὕνεκα ( = οὗ ἕνεκα) and ὁθούνεκα ( = ὅτου ἕνεκα) because, εὖτε since (poetic and Ionic; also temporal), and by ὅπου since (Hdt. 1.68, X. C. 8.4.31, I. 4.186). Homer has ὅ or ὅ τε because.

b. ὡς frequently denotes a reason imagined to be true by the principal subject and treated by him as a fact (2241). ὅτι often follows διὰ τοῦτο, διὰ τόδε, ἐκ τούτου, τούτῳ. διότι stands for διὰ τοῦτο, ὅτι. ὅτε and ὁπότε usually mean when (cp. cum); as causal conjunctions they are rare, as ὅτε τοίνυν τοῦθ᾽ οὕτως ἔχει since then this is the case, D. 1.1, ““χαλεπὰ . . . τὰ παρόντα ὁπότ᾽ ἀνδρῶν στρατηγῶν τοιούτων στερόμεθα” the present state of affairs is difficult since we are deprived of such generals” X. A. 3.2.2. Causal ὅτε, temporal ὅτε rarely, can begin a sentence. When they approach the meaning if, ὅτε and ὁπότε take μή. In Attic prose inscriptions ἐπεί is rare, διότι does not occur, and ὧν ἕνεκα is generally used for διόπερ.