An adverbial clause performs the function of an adverbial modifier. It can modify a verb, an adjective or an adverb in the principal clause.
He stopped as Kravat came rushing out. (Heym)
He was getting on better than he’d expected. (Lindsay)
Frank... returned to the auction room as fast as his legs would carry him.
According to their meaning we distinguish the following kinds of adverbial clauses: adverbial clauses of time, place, cause (reason), purpose, condition, concession, result, manner, and comparison.
Adverbial clauses are joined to the principal clause by means of subordinating conjunctions; they are not joined to the principal clause asyndetically except sometimes adverbial clauses of condition.
An adverbial clause may precede the clause to which it is subordinated or follow it. In the first case it is separated from the principal clause by a comma, in the second, as a rule, no comma is used. An adverbial clause may also interrupt the principal clause, in which case acomma is used at the beginning and at the end of it.
When things are at the worst, they’re sure to mend. (Collins)
He was sipping the whisky and soda when she returned. (Cronin)
The first words, when we had taken our seats, were spoken by my lady.
If we have two or more homogeneous adverbial clauses they are separated from each other by a comma.
He (Oliver) was still the same gentle, attached, affectionate creature that he
had teen when pain and suffering had wasted his strength, and when he was
dependent for every slight attention and comfort on thosewho tended him.