These two words in English seem to be the same word in Spanish, so there can be some confusion. Let’s clear that up.
Why is used to ask questions:
Why are you here?
Why is the sky blue?
It is also used as a relative pronoun:
I wonder why she goes out with him.
That’s why I refuse to watch “Avatar”. (Not *That’s because I…!)
Because is used in answering the questions asked by why. Here, it’s a conjunction, followed by a subject and verb:
I’m here because you asked me to be.
It’s blue because air scatters blue light better than red.
Also, when combined with “of”, it becomes a preposition followed by a noun with no verb, with a meaning similar to “due to”:
I’m here because of Bill.
It’s blue because of the scattering of blue light.
Answer a question *