An adverb may be a single word such as quickly, here or yesterday (see the page Adverbs), or a phrase such as the day before yesterday or to see my mother (see the page Adverb Phrases). However, adverbs can also be clauses, containing a subject and a full verb. This page will explain the basic types of adverb clauses (sometimes called "adverbial clauses") and how to recognize them.
Adverbs, adverb phrases, and adverb clauses
Look at these sentences:I saw the movie yesterday.
I saw the movie on Friday.
I saw the movie before I left for Calgary.
In the first sentence, "yesterday" is a one-word adverb, "on Friday" is an adverb phrase, and "before I left for Calgary" is an adverb clause. All of them answer the question "When?", but the adverb clause has a subject ("I") and a full verb ("left"). It is introduced by "before", so it is a dependent clause. This means that it cannot stand alone: "Before I left for Calgary" would not be a full sentence. It needs a main clause ("I saw the movie"). An adverb clause, then, is a dependent clause that does the same job as an adverb or an adverb phrase.