since的用法：
（1）Since the rain has stopped,let’s go for a walk.
既然雨停了，我們出去散散步吧。
（2）Since we are young,we shouldn’t be too afraid of making mistakes.
因為我們還年輕，所以不應該太害怕犯錯誤。
（3）Since you are free today, you had better help me with my mathematics.
既然今天你休息，你最好幫我補習數學。
because的.用法：
（1）We couldn't go out because these robber are still outside.
我們不能出去，因為這些強盜還在外面。
（2）I do it because I like it.
我做這件事是因為我喜歡。
（3）You can trust those products because the quality never varies.
你可以信賴那些產品，因為它們的質量從來不變。
（4）We went by bus because it was cheaper.
我們乘公交汽車去，因為這樣便宜一些。