An Advervb Clause is a group of words that functions as an Adverb in a sentence. The Clauses can modify other words in the sentence, such as adjectives, verbs or adverbs. In general they add some information about the words or about the conditions of an action. The Adverb Clauses are flexible and can be placed at the beginning, middle or end of a sentence. So, a subordinating adverb phrase that comes before the main clause is followed by a comma.