When you combine sentences with a conjunction, you must put a comma before the conjunction.

Let's fix those incorrect sentences from the main page.

Incorrect: Mary gave Jim some eggs, because he was hungry.
Correct:  Mary gave Jim some eggs because he was hungry.
This sentence doesn't need a comma because it only expresses one complete thought, and it doesn't contain a conjunction.  Because is not a conjunction. If you don't believe me, check the list.

Incorrect: Mary gave Mike some eggs, and some money.
Correct:  Mary gave Mike some eggs and some money.
This sentence does not need a comma because it only expresses one complete thought.  The conjunction and is not joining two otherwise complete sentences.  It is true that Mary gave Mike two things, but only one complete thought is expressed.

Incorrect: Mary is nice, she is also pretty.
Correct:  Mary is nice, and she is also pretty.
This sentence requires a comma and a conjunction because it joins two complete thoughts:
Mary is nice.  She is also pretty.
Both of these are sentences by themselves.  Remember the rule:

When you combine sentences with a conjunction, you must put a comma before the conjunction.

Incorrect: Mary can be quite contrary_but she isn't that way very often.
Correct: Mary can be quite contrary, but she isn't that way very often.
This sentence does require a comma because it joins two complete thoughts:
Mary can be quite contrary.  She isn't that way very often.
Both of these are sentences by themselves.

When you combine sentences with a conjunction, you must put a comma before the conjunction.

Got it?

Let's find out.