An adverb clause is a group of related words that have at least a subject and a verb, and whose function is to modify or add detail to adjectives, verbs or other adverbs. This type of clauses often answers why, how, when, where or in what circumstances something is done or takes place, and they begin with subordinating conjunctions like because, since, as, after, when, before an once.

In the sentence, then, the adverb clause is “because he forgot that the burner was still on” and the word it modifies is the main action (the verb): “burned” because it provides information of why the child burned his hand.