When to use a comma before a conjunction

When to use a comma before a conjunction.

1. When two simples sentences are joined together using a conjunction, it should be preceded by a comma. (This is also covered in the lesson "Commas".)

Examples:

Our team was given 2 minutes' notice, but theirs had been training for weeks.

("Our team was given 2 minutes' notice." + "Theirs had been training for weeks". These two sentences are merged into one using the conjunction "but".
In this role, "but" should be preceded by a comma.)

I have spoken to Sarah, and she has confirmed the delivery date.

("I have spoken to Sarah." + "She has confirmed the delivery date".
The conjunction "and" should be preceded by a comma.)

I have spoken to Sarah, and confirmed the delivery date.

("I have spoken to Sarah." + "confirmed the delivery date". The conjunction
"and" should not be preceded by a comma in this example, because
"confirmed the delivery date" is not a sentence. This is a list comprising two
actions: "spoken to Sarah" and "confirmed the delivery date".)

We eventually found the cave, but Jack was not there.

We built the hut in an hour and then painted it.

COMPOUND SENTENCE?

A sentence made up of two simple sentences is called a "compound sentence". The conjunction that joins the two halves of a compound sentence should be preceded by a comma.

I love milk. (< normal sentence)

I cannot drink it when it is
warm.(< normal sentence)

I love milk, but I cannot drink it when it is warm. (< compound sentence)

Note the comma before "but".

Associated lessons:

What are conjunctions?
Conjunctions and semicolons