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1. because引导的原因状语从句一般放于主句的后面，because从句位于句首时要用逗号分开，放在句末时，可不用逗号分开。because表示直接原因， 语气最强， 最适合回答why引导的疑问句。because of 也表示原因，但它后面不接从句, 只能接名词, 代词或动名词。注意：because 和so 不可同时出现在一个句子里。
I do it because I like it. = I like it so I do it.
We went by bus because it was cheaper. = It was cheaper so we went by bus.
He can’t go to school because of his illness.

2. since引导的原因状语从句一般放于主句之前表示已知的、 显然的理由(通常被翻译成“既然”= now that )， 较为正式， 语气比because弱。
Since you are free today, you had better help me with my mathematics.
Since you don't trust him, you should not employ him.
Now (that) you are grown up, you should not rely on your parents.

3. as 引导原因状语从句时表示附带说明的“双方已知的原因”，含有对比说明的意味，语气比since弱， 较为正式， 位置较为灵活(常放于主句之前)。
As it is raining, you’d better take a taxi.
As you are tired, you had better rest.
I went to bed early, as I was exhausted.

4. for引导的是并列句表示原因但并不说明主句行为发生的直接原因， 只提供一些辅助性的补充说明， for引导的并列句只能放于主句之后并且必须用逗号将其与主句隔开。
He could not have seen me, for I was not there.
He seldom goes out now, for he is very old.

来源于：[url]https://www.hjenglish.com/new/p1207095/[/url]

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