comma splice / run-on sentence
Independent clauses joined by a comma alone or by nothing at all.
Do not join independent clauses with a comma alone: The attachment is missing, please send it again. This error is a comma splice. Correct it in one of three ways.
Use a period: The attachment is missing. Please send it again.
Use a semicolon: The attachment is missing; please send it again.
Keep the comma and add a coordinating conjunction (and, but, or, nor, for, so, yet): The attachment is missing, so please send it again.
When independent clauses have no punctuation and no conjunction between them, the error is a fused sentence: The attachment is missing please send it again. The same corrections apply. Comma splices and fused sentences are both run-on sentences.
Examples
Use The payment cleared, so we released the order. (A comma and the coordinating conjunction so join the two independent clauses.)
Use The front entrance was locked; we used the side door. (A semicolon joins two closely related independent clauses.)
Use The figures were incomplete. We postponed the announcement. (A period separates the two independent clauses.)
Avoid The reservation was confirmed, no deposit was required. (A comma cannot join two independent clauses by itself; this error is called a comma splice.)
Avoid The printer stopped halfway through the report we called a technician. (Two independent clauses run together with no punctuation or conjunction; this error is called a fused sentence.)
A comma before then does not join two independent clauses, because then is an adverb, not a coordinating conjunction. We sealed the crates, then we loaded the truck is a comma splice. Many writers punctuate this way, but it does not belong in standard English. Write We sealed the crates, and then we loaded the truck, or We sealed the crates; then we loaded the truck. With one subject, the comma creates no splice, but then still cannot join the two verbs by itself. Write We sealed the crates and then loaded the truck, not We sealed the crates, then loaded the truck.