Words introducing adverb clauses

Subordinating conjunctions: after, although, as, as if, because, before, even though, if, in order that, since, so that, than, that, though, unless, until, when, where, whether, while

Words introducing adjective clauses

Relative pronouns: that, which, who, whom, whose

Relative adverbs: when, where, why

Words introducing noun clauses

Relative pronouns: that, which, who, whom, whose

Other pronouns: what, whatever, whichever, whoever, whomever

Other subordinating words: how, if, when, whenever, where, wherever, whether, why