1. In a Simple List:
* Use a comma before "and" when listing three or more items.
* Example: I bought apples, oranges, and bananas.
* Do not use a comma before "and" when listing only two items.
* Example: I bought apples and oranges.
2. In Compound Sentences:
* Use a comma before "and" when joining two independent clauses (complete sentences).
* Example: I went to the store, and I bought groceries.
* Do not use a comma before "and" if the two clauses are very short and closely related.
* Example: I went to the store and bought groceries.
3. Other Considerations:
* Use a comma before "and" for clarity if the sentence is long or complex, even if it's not a formal compound sentence.
* Example: I went to the store, which was crowded, and I bought groceries, which were expensive.
Remember: These are general guidelines. There are always exceptions based on the nuances of individual sentences. It's often helpful to read your writing out loud to see if the comma placement sounds natural.