I'm not a teacher, but I'm a business writer. I wanted to throw one more wrinkle in: the comma before the "because" if the first part is in the negative.
I didn't hire him because he was so good looking...
Compare: I didn't hire him, because he was so good looking that I was afraid my colleagues would just stare at him all day and not get any work done. With the comma, in fact, I didn't hire him, and here's why.
And: I didn't hire him because he was so good looking but because he really knows his stuff. Without the comma, in fact, I did hire him, but not for the reason stated before the because.
I just wanted to throw this monkey wrench in. I'll let greater minds with more pedagogy skills handle the case in the positive.