Adverbial clause

An adverbial clause is a clause that functions as an adverb. In other words, it contains a subject (explicit or implied) and a predicate, and it modifies a verb.

§  I saw Joe when I went to the store. (explicit subject I)

§  He sat quietly in order to appear polite. (implied subject he)

According to Sidney Greenbaum and Randolph Quirk, adverbial clauses function mainly as adjuncts or disjuncts. In these functions they are like adverbial phrases, but due to their potentiality for greater explicitness, they are more often like prepositional phrases (Greenbaum and Quirk,1990):

§  We left after the speeches ended. (Is it an adverbial clause, adverbial phrase, or prepositional phrase?)

§  We left after the end of the speeches.(Is it an adverbial clause, adverbial phrase, or prepositional phrase?)

Contrast adverbial clauses with adverbial phrases, which do not contain a clause.

http://en.wikipedia.org/wiki/Adverbial_clause