Sentence Structure
Compound sentences
Use a comma before the conjunction when joining two full clauses — or use a semicolon if the ideas are closely linked.
Why this happens
Compound sentences join two or more independent clauses with coordinators (and, but, or, so, yet, for, nor). Errors happen when writers comma-splice without a conjunction, use the wrong coordinator for the logic, or join clauses that are not truly independent. Strong writers match the conjunction to the relationship: contrast (but), result (so), addition (and).
See it in a sentence
IncorrectThe lecture was clear, I still took notes.
CorrectThe lecture was clear, but I still took notes.
Teaching note: Add a coordinating conjunction or use a semicolon — a comma alone is not enough.
IncorrectShe studied all week, so but she felt unprepared.
CorrectShe studied all week, but she still felt unprepared.
Teaching note: Use one coordinator per join — so but is not valid.
IncorrectThe printer jammed and because the paper was wet.
CorrectThe printer jammed because the paper was wet.
Teaching note: Because introduces a dependent clause — not a second independent clause for and.
More examples
The sun set; the air cooled quickly.
A semicolon can join two closely related independent clauses without a coordinator.
Not only did costs rise, but profits also fell.
Correlative pairs (not only... but also) can build compound structures with inverted word order.
I wanted to stay, yet I left early.
Yet shows contrast, similar to but — slightly more formal or emphatic.
Join two short clauses: She laughed, he sighed.
Even short clauses need proper punctuation if both are independent.
Quick check
The evidence is strong, therefore the claim holds.
Show the fix
The evidence is strong; therefore, the claim holds. Therefore between independent clauses needs a semicolon (or period), not only a comma.
Practice this topic in Wrytesmart
Open the grammar workspace to complete interactive exercises for compound sentences. You will spot errors, fill blanks, and transform sentences — always with teaching notes that explain the principle, not just the correction.
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