What is a compound sentence?
A compound sentence joins two or more independent clauses — each of which could stand alone as a sentence — using a comma plus a coordinating conjunction, a semicolon, or a colon. “The rain stopped, and we went outside” is compound: both halves are complete sentences.
The four sentence types
English sentences are classified by how many independent and dependent clauses they contain. An independent clause has a subject and a verb and expresses a complete thought; a dependent clause has a subject and a verb but cannot stand alone, because a subordinating word — although, because, when, if, since — leaves it hanging.
A simple sentence has one independent clause and no dependent ones: “The rain stopped.” A compound sentence has two or more independent clauses: “The rain stopped, and we went outside.” A complex sentence has one independent clause plus at least one dependent clause: “When the rain stopped, we went outside.” A compound-complex sentence has at least two independent clauses and at least one dependent clause: “When the rain stopped, we went outside, but the ground was still wet.”
The distinction is worth learning for one practical reason: the punctuation rules differ, and most comma errors come from applying the wrong type's rule.
How to punctuate a compound sentence
There are three legitimate joins. A comma followed by a coordinating conjunction — and, but, or, nor, for, so, yet — which is the default. A semicolon with no conjunction, when the two clauses are closely related and you want them to feel like one thought. Or a semicolon followed by a conjunctive adverb such as “however” or “therefore”, which takes a comma after it: “The rain stopped; however, the ground was still wet.”
What you cannot do is join two independent clauses with a comma alone. That is a comma splice, and it is the most commonly marked error in student writing. Nor can you run them together with no punctuation at all — a fused sentence. Both are forms of run-on, and both have easy repairs.
Incorrect:
The rain stopped, we went outside.Correct: The rain stopped, and we went outside.
Comma splice. Add a coordinating conjunction, or upgrade the comma to a semicolon.
Incorrect:
The rain stopped we went outside.Correct: The rain stopped; we went outside.
Fused sentence. A semicolon suits closely linked clauses.
Incorrect:
The rain stopped, however the ground was still wet.Correct: The rain stopped; however, the ground was still wet.
“However” is a conjunctive adverb, not a coordinating conjunction — it cannot join clauses with a comma.
When to use one
Compound sentences give two ideas equal weight. That is their whole rhetorical function, and it is why they suit contrast and sequence: two findings that matter equally, two events in order, a claim and its counterweight.
When the ideas are not equal — when one explains, qualifies or gives the timing of the other — a complex sentence is the better choice, because subordination shows which idea is the main one. “Although the rain stopped, the ground was still wet” tells the reader what to focus on; the compound version leaves them to work it out.
A paragraph built entirely from compound sentences develops a characteristic drone, everything joined by “and” and “but”, nothing ranked. Mixing the four types is what produces prose with a shape.
