The children learned to ride their bicycles, and then they rode them in the park.
I want to see that new movie, but I am too late.
Sammi wants to get a new haircut, so we are going to the salon on Friday.
If you're not sure whether or not the sentence needs a comma, check to see if each half of the sentence makes sense by itself. You can test whether each half makes sense by replacing the conjunction with a period. If each half does make sense by itself, you need to add a comma before the conjunction. Compound subjects and compound predicates are not separated by commas.
I visited my sister in London and had a wonderful time.
This sentence doesn't need a comma because the second half of the sentence can't stand alone: had a wonderful time isn't a complete sentence because the subject is missing. This is an example of a compound predicate (visited my sister + had a wonderful time), which doesn't require a comma.
I meant to return that book to you but forgot it on the kitchen table.
Just like the previous sentence, this one doesn't need a comma because forgot it on the kitchen table is missing the subject and doesn't make sense by itself.
Practice What You've Learned
Exercises are reserved for account holders. Please log in.