since的用法：
（1）Since the rain has stopped,let’s go for a walk.
既然雨停了，我们出去散散步吧。
（2）Since we are young,we shouldn’t be too afraid of making mistakes.
因为我们还年轻，所以不应该太害怕犯错误。
（3）Since you are free today, you had better help me with my mathematics.
既然今天你休息，你最好帮我补习数学。
because的.用法：
（1）We couldn't go out because these robber are still outside.
我们不能出去，因为这些强盗还在外面。
（2）I do it because I like it.
我做这件事是因为我喜欢。
（3）You can trust those products because the quality never varies.
你可以信赖那些产品，因为它们的质量从来不变。
（4）We went by bus because it was cheaper.
我们乘公交汽车去，因为这样便宜一些。