because和as

1）because语势最强，用来说明人所不知的原因，回答why提出的问题，是句子中的重要部分，常放在句末，不能用其它三个替换。逗号可加可不加。

You can trust those products because the quality never changes.

I haven't been to the cinema lately, because I am too busy.

I didn't go, because I was afraid.

2）当原因是显而易见的或已为人们所知，就用as或 since。since稍正式一些。
Since /As the weather is so bad, we have to delay our journey.