The interrogative pronouns are: who, which, what ([§§ 163–165]).
The interrogative adverbs are: where, when, whence, whither, how, why.
The most important subordinate conjunctions are: because, since (= because), though, although, if, unless, that (in order that, so that), lest, as, as if, as though, than, whether (whether ... or).
378. According to their use as parts of speech, subordinate clauses are adjective, adverbial, or noun clauses.
I. ADJECTIVE CLAUSES
379. A subordinate clause that modifies a substantive is called an adjective clause ([§ 47]).
- {Able men | Men of ability | Men who show ability} can always find employment.
- {Treeless spots | Spots without trees | Spots where no trees grew} were plainly visible.
In each of these groups, a noun (men, spots) is modified (1) by an adjective, (2) by an adjective phrase, (3) by an adjective clause. The sense remains unchanged.
380. Adjective clauses may be introduced (1) by relative pronouns, (2) by relative adverbs of place (where, whence, whither, etc.) or time (when, while, etc.).