第1个回答 推荐于2016-04-22
1. Becasue：当原因作为句子或陈述最重要的一部分时，我们使用because.
Because通常出现在句首部分。
Because is used when the reason is the most important part of the sentence or utterance. The because clause usually comes at the end:
e.g. I went to Spain last summer because I wanted the guarantee of sunshine on every day of my holiday.
2. As和since：当原因是众所周知时，我们使用as和since，因此，这样的原因一般不那么重要。
as或since从句一般置于句首。
As and since are used when the reason is already well known and is therefore usually less important. The as or since clause is usually placed at the beginning of the sentence:
e.g. As the performance had already started, we went up to the balcony and occupied some empty seats there.
e.g. Since John had already eaten, I made do with a sandwich.
3. For：For意味着原因是事后给出的。
For从来都不会置于句首，带有书面英语而非口语的色彩。
For suggests that the reason is given as an afterthought. It is never placed at the beginning of the sentence and is more characteristic of written, rather than spoken English:
e.g. I decided to stop the work I was doing - for it was very late and I wanted to go to bed.本回答被提问者和网友采纳