What is the adverb clause in I will call as soon as he comes in.

The correct answer is "as soon as he comes in."

Adverb clauses are chains of words that work together as an adverb. They can modify a verb, an adjective, another adverb, or a phrase. They give extra information, such as when, why, where and how. They possess a subject and a predicate, and start with a subordinating conjunction like as, after, and unless.

In this example, the adverb clause modifies the verb call.