hello,
An adverbial clause is a clause that functions as an adverb.
let's try this example:
"I washed the dishes before I left home."
as you would motice, the clause "before I left home" is an adverbial clause that modifies or clarifies the manner of the verb, "washed". here, the adverbial clause has subject ("I") and a full verb ("left") but it is introduced by "before", hence an independent clause - which means, that clause can not stand alone.
here's a tip to recognize a clause:
most adverb clauses can be recognized because they are introduced by a particular word or phrase. These words and phrases are called subordinating conjunctions:
after, before, until, while, because, since, as, so that, in order that, if, unless, whether, though, although, even though, where.
ex:
"She took a computer course so that she could get a better job."
the adverb clause here is "so that she coould get a better job."
try this site too:
http://www.yourdictionary.com/dictionary-articles/Teaching-Adverbial-and-Adjective-Clauses.html
:)