A complete sentence is a group of words that expresses a complete thought. It typically contains a subject, which tells who or what the sentence is about, and a predicate, which tells what the subject does or is. For example, in the sentence "The dog barks," "The dog" is the subject, and "barks" is the predicate. Complete sentences can also include additional elements like objects or modifiers to provide more detail. For instance, "The dog barks loudly at the mailman" includes the subject, predicate, and additional information about how and at whom the dog barks.