The document discusses adverb clauses, which are subordinate clauses that modify verbs, adjectives, or other adverbs. Adverb clauses answer questions like how, where, when, why, to what extent, or under what condition. They contain both a subject and a verb, unlike adverb phrases. Common subordinating conjunctions that introduce adverb clauses include after, although, as, before, since, when, and while. Adverb clauses can appear anywhere in a sentence and are usually set off by commas when beginning a sentence.