as、because、for与since这些连词均含有“因为”之意。
as ：从属连词，语意较because, since弱，着重在主句，所表示的原因或理由是“附带的”。
- As you were out, I left a message.
你不在，所以我留了一张字条儿。 - She may need some help as she's new.
她是新来的，可能需要一些帮助。
because ：从属连词，语意强，着重直接的原因或理由，所引出的从句是全句的重心，用以回答why提出的问题。
- I did it because he told me to.
是他吩咐我才做的。 - Just because I don't complain, people think I'm satisfied.
就因为我不发牢骚，大家便以为我满意了。
for ：并列连词，语气较弱，所引出的句子一般放在后面，表示附带说明的理由或推断理由。
- The town is famous for its cathedral.
这个城镇以大教堂著名。 - She gave me a watch for my birthday.
她送给我一块手表作为生日礼物。
since ：语意比because弱，但比as强，一般用于表示“由于大家已知的事实”。
- We thought that, since we were in the area, we'd stop by and see them.
我们想，既然到了这个地方，就该顺便去看看他们。
