(1) Meanwhile her father has been saving money, because it’s the duty of the bride’s family to provide a home for the new couple,… (Passage A, Unit 6)
(2) As she grew up, she herself quite possibly used needle and thread to help her mother and grandmother slave over the task of sewing enough cotton items to fill a large chest, for, at her marriage she is expected to provide as many as 100 sheets,… (Passage A, Unit 6)
(3) And since she cleaned many houses each week, we had a great many of them. (Passage A, Unit 5)
(4) He fell asleep on the sofa, as he was so tired.
上面四例中分别包含英语中较常用的表示原因的连词because, for, since, as。这些连词在意义和用法上都不尽相同。在用法方面，because, since, as都是从属连词，用来引导原因状语分句；而for是并列连词，用来连接分别表示结果和原因的两个并列分句。
because是这四个连词中语势最强的一个，它表示的原因通常是听话人所不知道的，而且通常是造成某一结果的直接原因， 因此由它所引导的原因状语分句往往是句中最重要的部分，于是也就经常被放在句末（当然有时也可放在句首）。例如：
He sold his car because it was too small.
She took off her jacket because she felt hot.
I was late because I missed the bus.
I didn’t buy the TV set because it was too expensive.
Because he was ill, he didn’t come to the class.
Because it was raining, I didn’t go out for a walk.
since和as区别不大（since比as略为正式），通常表示听话人已知的原因。带有since / as的句子的侧重点往往在于结果，而原因只是附带说明。由since / as引导的原因状语分句经常放在句首（也可放在句末）。例如：
Since you say so, l must believe it.
Since you refuse to give advice, I shall be forced to ask Mr. Jackson for advice.
As you won’t help, I have to do the job myself.
As you didn’t answer my letter, I wrote again.
for后面的分句通常表示间接的原因，有时是对造成某一结果的原因作补充说明(1)，有时是为所作的推断或预见提供理由(2)。For后面的分句只可放在句末，前面通常用逗号或破折号与前一分句隔开。例如：
(1) He is absent today, for he is ill.
I decided to stop and have lunch — for I was feeling hungry.
(2) It must have rained last night, for the ground is wet.
比较：The ground is wet, because it rained last night.
Someone must have entered my room, for my money is missing.
It will rain, for it’s getting dark.
He will certainly succeed, for he works so hard.
请提示学生注意：在下列情况下，只能用because，不能用其他连词。
1. 在回答以why开头的问题时。例如：
A: Why can’t I go?
B: Because you are too young.
[误] Since / As / For you are too young.
2. 在only, just, simply等副词之后，以及在否定词not或not…but…结构之后。例如：
She did so much for her son only because she loved him.
He got angry just because someone spoke ill of him.
You shouldn’t do anything simply because other people are doing it.
I did it, because I have to, not because I want to.
I like the job, not because it’s easy, but because it’s difficult.
3. 在it is之后。例如：
If you feel cold, it is because you didn’t put on your overcoat.
It was because he was ill that he didn’t come to the class.
表示原因的连词语法
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