A sentence is a group of words that expresses a complete thought. It usually contains a subject, which tells us who or what the sentence is about, and a predicate, which tells us what the subject does or is. For example, in the sentence “The dog barks,” “The dog” is the subject, and “barks” is the predicate. Sentences can be simple or complex. A simple sentence has just one subject and one predicate, while a complex sentence can have multiple subjects and predicates. For instance, “The cat sleeps on the couch” is simple, while “The cat sleeps on the couch, and the dog plays in the yard” is complex.