3. 在强调结构It is/was that和关联词not but引导的原因状语从句中，宜用because。It’s because he helped you that I’m prepared to help him. [/PRE]正是因为他帮助过你，所以我乐意去帮助他。[/PRE]
He decided to give up the chance of going abroad, not because he did not want to but because his wife was ill.
他决定放弃出国的机会，不是因为他不想去，而是因为他妻子病了。
B. since
since 因为，既然。引导的从句大多置于句首，主从句的时态一般相同。
Since you have no licence, you are not allowed to drive.
因为你没有驾驶执照，所以不允许你开车。
Since you are all here, let’s try and reach a decision.
既然大家都来了，咱们就设法作出一个决定吧。
C. as
as 由于。一般多用于句首。
As she was ill, she didn’t come to the party.
由于病了，她没来参加晚会。
As he was not well enough, I had to go without him.
由于他身体欠佳，我只好不带他去了。
As it rained, we all stayed at home.
由于下雨，我们都呆在家里。
D. because, since, as 的区别
1. because语气最强，表达的是未知的新信息，一般置于主句之后，也可以放在主句之前，用逗号隔开。在回答“why”引导的特殊疑问句时，或在强调结构It is/was that和关联词not but引导的原因状语从句中，要用because。另外，because还常和副词just, merely（单；只；纯粹）等连用。
2. since 往往表示的是已知的客观事实，或分析后的推理，引导的从句大多置于句首，主从句的时态一般相同。
3. as 表示的理由最弱，只是对主句的附带说明，重点在主句。as从句通常放在主句前。
Just because he doesn’t complain, you must not suppose that he is satisfied.