A group of words which forms part of a sentence is a clause if it contains

A clause is a group of words that contains a subject and a predicate and can express a complete thought. A subject is the noun or pronoun that performs the action or is being described in a sentence, while a predicate is the part of a sentence that includes the verb and all the words that follow it. For example, in the sentence "I like to read books," the group of words "I like to read books" is a clause because it contains a subject "I" and a predicate "like to read books" that expresses a complete thought. However, in the sentence "Reading books," the group of words "Reading books" is not a clause because it is missing a predicate.