Every sentence has two essential parts: a subject (who or what the sentence is about) and a predicate (what the subject does or is). For example, in "The dog barked loudly," "The dog" is the subject and "barked loudly" is the predicate. Simple enough — but sentences quickly grow more complex when you add objects, phrases, and clauses.
Objects receive the action of the verb. In "She kicked the ball," "the ball" is the direct object because it receives the action of "kicked." An indirect object tells us to whom or for whom the action is done: "She gave him the ball" — "him" is the indirect object. Complementing the subject or object, subject complements follow linking verbs: "She is a doctor" — "a doctor" is a subject complement.
Clauses are groups of words with a subject and a verb. An independent clause can stand alone as a sentence: "I went to the store." A dependent (subordinate) clause cannot stand alone because it begins with a subordinating conjunction like "because," "although," "when," or "if": "because I needed milk." Dependent clauses act as nouns, adjectives, or adverbs within larger sentences.
Sentences are classified by their clause structure. A simple sentence contains one independent clause: "The cat slept on the mat." A compound sentence joins two independent clauses with a coordinating conjunction (FANBOYS: for, and, nor, but, or, yet, so): "The cat slept on the mat, and the dog lay by the door." A complex sentence combines one independent clause with at least one dependent clause: "The cat slept on the mat while the dog lay by the door." A compound-complex sentence has at least two independent clauses and one dependent clause: "While I was cooking, the phone rang and the dog barked."
A common error is creating sentence fragments — incomplete sentences that are missing a subject, a verb, or a complete thought. "Running through the park." is a fragment because it lacks a subject. "Because she was tired." is a fragment because the dependent clause cannot stand alone. Another common error is the run-on sentence, where two independent clauses are joined without proper punctuation or conjunctions: "I went to the store I bought milk" should be "I went to the store, and I bought milk."