because强调原因，在最开头使用because说明你想额外强调原因

　　I'm tired because I didn't sleep very well.因为我没有睡好，所以我很累。

　　Because my bed is uncomfortable, I'm getting a bad back.

　　因为我的床不好睡，我的背越来越不好了。

　　 Because it is an area of outstanding natural beauty, the number of boats available for hire on the river is limited. 这是一个自然风景极美的地方，所以河上可供租用的船只数量有限。

　　而as和since比because更正式，我们用这两个词时，更强调结果而非原因。

　　I hope Tom's brought that comic as I wanted to borrow it from him.我希望Tom把那本漫画书带来，因为我想要向他借那本书。

　　She may need some help as she's new. 她是新来的，可能需要一些帮助。

　　Noodles are popular since they're easy to cook.面条很受欢迎，因为它们容易煮。

　　 I'm forever on a diet, since I put on weight easily.  我一直在节食，因为我的体重很容易增加。