The mood of a subordinate clause which is intimately connected with the thought of the clause on which it depends, is often assimilated to the mood of that clause. Such subordinate clauses may be simply dependent or sub-dependent ( cross2180).
a. This idiom is most marked in Unreal and Less Vivid Future conditions where the mood of the protasis is the same as that of the principal clause. It is also very common when a past indicative or an optative attracts the mood of a subordinate clause introduced by a relative word referring to indefinite persons or things or to an indefinite time or place. But subordinate clauses standing in a less close relation to the main clause, because they do not continue the same mental attitude but present a new shade of thought, retain their mood unassimilated; e.g. a relative clause, or a temporal clause expressing purpose, after an unreal condition may stand in the optative (
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].