A phrase is a group of words that does not contain both a subject and a verb. It can function as a noun, adjective, or adverb but does not express a complete idea on its own. There are four main types of phrases: noun phrases, verb phrases, adjective phrases, and adverb phrases. A sentence contains both a subject and a predicate and can stand alone, while clauses contain a subject and verb but may or may not be able to stand alone. Subject-verb agreement requires that the verb matches the subject in number - singular verbs must go with singular subjects and plural verbs with plural subjects.