An adverbial clause is an English grammatical construction in which a subordinate clause in a sentence modifies the verb of a main clause. By doing this, the clause essentially plays the same role as an adverb does in a sentence. In most cases, these clauses are begin by a subordinate conjunction and often show either the time when or the reason why that the verb occurs. As with subordinate clauses, an adverbial clause includes a noun and a verb but cannot stand alone in a sentence like the main clause can.
Sentence structure in the English language is often distinguished by different groups of words that serve a purpose to the overall meaning of the sentence. A clause in a sentence contains a noun and a verb but it may or may not be able to stand alone. In some cases, these clauses, much like a single word, can have the function of a part of speech in the sentence. One of these functioning clauses is an adverbial clause, which behaves in the same manner as an adverb does in a sentence.
In many cases, an adverbial clause is used to modify the main verb in terms of explaining when the action takes place. For example, imagine the sentence, "After we took the kids to school, we went back at home and rested." With this sentence, "After we took the kids to school" acts as the adverb, since it explains when the subjects of the sentence went back home and rested.
Another common use for an adverbial clause is to show cause, or, in other words, to explain why the main verb in the sentence is taking place. For an example of this type, imagine the sentence, "Since I wasn't making enough money, I decided to quit my job." In this case, the adverbial clause at the beginning of the sentence explains why the subject decided to quit his job, creating a cause and effect in the sentence.
It is important to note in both examples, as in all adverbial clauses, that the subordinate clauses could not stand alone and still make sense as a sentence. This sets them apart from the main clauses. Both main clauses in the examples, "We went back home and rested" and "I decided to quit my job," could stand alone as sentences and make perfect sense. The adverbial clauses are thus the subordinate clauses, modifying the main clauses and providing a bit more information to complete the thoughts.