Here's the rule:
Independent Clause + Comma + Conjunction + Independent Clause
Example:
* I went to the store, and I bought some milk.
Here's why:
* Independent clauses are complete sentences that can stand alone.
* Conjunctions are words that connect words, phrases, or clauses. Common conjunctions include *and, but, or, so, yet*.
When you don't need a comma:
* If you are connecting a dependent clause to an independent clause, you don't need a comma.
* If you are connecting two words or phrases, you don't need a comma.
Example:
* I went to the store because I needed milk. (Dependent clause + independent clause)
* I bought milk and bread. (Two phrases)
Let me know if you have any other questions about commas!