We talked awhile back about using coordinating conjunctions to join two simple sentences. When you use a comma and one of the FANBOYS to join two simple sentences, you make a compound sentence.
The word but is a coordinating conjunction; but is the B word in FANBOYS.
Anytime you use but to join two complete thoughts, you need a comma in front of but.
Look at these examples with a comma in front of but:
I want to ride my bike, but it is too cold today.
My dad likes coffee, but he drinks it cold.
Jim said he would help me with my project, but his car broke down.
On the other hand, the word because is a subordinate conjunction, not a coordinating conjunction.
Do not use a comma in front of because.
Look at these complex sentences with because. There is no comma in front of because.
I wore my winter jacket today because it is cold.
I don’t like math because I can’t understand it.
Dad needs to scrape off his car because it snowed last night.
We use but to join two simple sentences into a compound sentence. We must use a comma with but.
The word because is part of a subordinate clause. Do not use a comma in front of because in a complex sentence.
Comma, but. No comma with because.