I have been asked for a little more explanation on the last post.
A dependent clause is dependent because it has a word out in the front of it that "introduces" it.
...He left. (a sentence, an independent clause)
...that he left (dependent clause)
...when he left (dependent clause)
...because he left (dependent clause)
...if he left (dependent clause)
When two (or more) dependent clauses are joined by a coordinate conjunction, it is very common for the introductory word for the second clause to be omitted. This is a standard pattern in English.
...when he arrived for the meeting and (when) he told us the news...
...because my aunt had physical problems and (because) she could not live alone...
...after we left the theater and (after) we walked to our car...
What happens here is that the second part of this compound LOOKS like an independent clause; i.e., it looks as if it is just a subject and verb by itself. You have to be aware of the construction in front of it and realize that it is really a dependent clause with the introductory word missing. And two dependent clauses that are joined by a coordinate conjunction do NOT take a comma.
Remember that the word "and" cannot link a dependent clause to an independent clause. So the second part HAS to be dependent.
I hope this helps.
Happy punctuating!
Margie