as和because作从属连词引导原因状语从句，意为“因为”。because引导的从句一般放主句后面，位于句首要用逗号分开。because表示直接原因，语气强，适合回答why引导的疑问句。as引导从句时表示附带说明的“双方已知的原因”，语气比why弱，位置较为灵活（常放于主句之前）。
1. because引导原因状语从句
（1）I do it because I like it.
我做这件事是因为我喜欢。
（2）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。
（3）We went by bus because it was cheaper.
我们乘公交汽车去，因为这样便宜一些。
“not...because”结构中的not否定的是because引导的整个从句。例如：
（1）The country is strong not just because it is large.
国强不在大。
（2）I didn't help him,not because I was unwilling,but because I was unable to do it.
我没有帮助他，不是因为我不愿意，而是因为我力不从心。
2. as引导原因状语从句
（1）As it is raining, you'd better take a taxi.
既然在下雨，你最好乘出租汽车。
（2）As you are tired, you had better rest.
既然累了，你最好休息一下。
（3）I went to bed early, as I was exhausted.
我睡得早，因为我已筋疲力尽了。
。