An adverbial clause is a dependent clause that functions as an adverb; that is, the entire clause modifies a verb, an adjective, or another adverb.s with all clauses, it contains a subject and predicate, although the subject as well as the (predicate) verb may sometimes be omitted and implied (see below). An adverbial clause is commonly, but not always, fronted by a subordinate conjunction —sometimes called a trigger word.