How to Use Commas to Join Two Complete Sentences
In English, when you join two complete sentences with the conjunctions and, or, but, nor, yet, so, or for, place a comma before the conjunction. If you use just a comma, you create a comma splice and your friends will all laugh at you. Examine these examples of complete sentences joined with a comma and a conjunction:
Agnes robbed the bank, and then she went out for a hamburger.
James spies, but apart from that lapse he is not a bad fellow.
Sam bribed the judges of this year’s state spitball contest, for he is determined to qualify for the national tournament.
Some sentences have one subject (who or what you’re talking about) and two verbs joined by and, but, or, and nor. Don’t put commas between the two verbs. You aren’t joining two complete sentences, just two words or groups of words. Here are some examples:
Wrong: Ella wrote a statement for the media, and then screamed at her press agent for an hour.
Why it is wrong: The sentence has one subject (Ella) and two verbs (wrote, screamed). You aren’t joining two complete sentences, so you shouldn’t place a comma before and. Either way, Ella should learn to control her temper.
Right: Ella wrote a statement for the media and then screamed at Larry for an hour.
Wrong: Larry has proposed a toast to his bride, but has given her nothing but a headache.
Why it is wrong: The sentence has one subject (Larry) and two verbs (has proposed, has given). The word but joins the two verbs, not two complete sentences. You don’t need a comma. Also, if she's putting up with Larry, she deserves a wedding gift.
Right: Larry has proposed a toast to his bride but has given her nothing but a headache.
Which sentence is correct?
A. Al slits envelopes with his teeth, but Dorothy opens the mail with a fork.
B. Al answers every letter on the day he receives it but doesn’t pay any bills.
Answer: Both sentences are correct. In sentence A, the conjunction but joins two complete sentences. A comma must precede the conjunction but. In sentence B, but joins two verbs (answers, does pay). No comma precedes the conjunction.