Simple, Compound, Complex and Compound-Complex Sentences Explained

Every sentence is built from clauses. A clause has a subject and a verb. An independent clause can stand alone as a sentence ("The dog barked"). A dependent clause can't — it starts with a word like because, although, when, if, which or that and leaves you waiting for more ("because the dog barked…").

Once students can spot those two kinds of clause, the four sentence structures are easy:

• Simple — one independent clause. "We ran fast." (It can still have two subjects or two verbs: "My brother and I played soccer.")
• Compound — two or more independent clauses joined by a comma + FANBOYS word (for, and, nor, but, or, yet, so) or by a semicolon. "I like tea, but she likes coffee."
• Complex — one independent clause plus at least one dependent clause. "Although it rained, we went outside."
• Compound-complex — at least two independent clauses and at least one dependent clause. "Although the hike was long, we reached the top, and the view was amazing."

The two-question test

Ask two questions about any sentence:

• How many independent clauses are there? (one, or two or more)
• Is there any dependent clause? (yes or no)

One independent and no dependent = simple. Two+ independent and no dependent = compound. One independent with a dependent = complex. Two+ independent with a dependent = compound-complex.

Common traps

• Compound verbs aren't compound sentences. "The cat jumped onto the table and knocked over a glass" has one subject doing two things — it's still simple.
• Phrases aren't clauses. "Running through the park, the girl lost her hat" begins with a phrase (no subject + verb of its own), so it's simple.
• A comma alone can't join two independent clauses. "I love reading, it relaxes me" is a comma splice. Fix it with a semicolon, a FANBOYS word, or a subordinating word: "I love reading because it relaxes me."

Teaching tip: have students underline independent clauses once and dependent clauses twice. The structure name falls out of the underlining.

Sentence purpose is a different question

Structure is about clauses. Purpose is about what the sentence does: declarative (makes a statement), interrogative (asks), imperative (gives a command) or exclamatory (shows strong feeling). A sentence has both — "Close the door before the cat gets out!" is complex in structure and imperative in purpose.