③or还是and

祈使句+or+陈述句前后是对立的

祈使句+and+陈述句前后是统一的

(or或and后的陈述句，常用一般将来时)

Study hard, or you won't pass the exam.

Study hard, and you will pass the exam.

两句都可以转换成“If条件句+主句”形式，注意：要去掉or和and：

If you don't study hard, you won't pass the exam.

If you study hard, you will pass the exam.

④because, since, as, for表示原因时的区别

尽管because, for, since, as都表示原因，但是用法上有不同.

because通常表示说话人认为这种理由或者原因是听话人所不知道的.

如:

He didn't attend the meeting because he had too much work to do.

since表示的原因是人们已知的事实，常译作“既然”。

如：

Since he can't answer this question, you'd better ask someone else.

as表示原因与since差别不大，只是语气更弱,多译为“由于”

如：

As I haven't seen the film, I can't tell you what I think of it.

for表示因果关系时，只能放在主句之后，不能放在句首。而且经常是对主句补充说明理由或推断原因。

如：

It rained last night, for the ground is wet.

⑤ as, when, while

这三个连词都可引导时间状语从句，但用法有所不同。

1) 当某事正在进行的时候，又发生了另一件事。While, when, as 都可用来引导表示“背景”的时间状语从句。例如：

As/When/While I was walking down the street I noticed a police car.

2) 当两个长动作同时进行的时候，最常用的是while。例如：

While mother was cooking lunch, I was doing my homework.

3) 当两个动作都表示发展变化的情况时，最常用的是as。例如：

As children get older, they become more and more interested in things around them.

4) 当两个短动作同时发生时，或表示“一边…一边…”时，最常用as。例如：

Just as he caught the fly, he gave a loud cry.

She looked behind from time to time as she went

5) 当从句的动作先于主句的动作时，通常用when。例如：

When he finished his work, he took a short rest.

6) 当从句是瞬间动作，主句是延续性动作时，通常用when。例如：

When John arrived I was cooking lunch.

(6)if, whether

if和whether都可作“是否”讲，在引导宾与从句是一般可互换。例如：

I wonder whether (if) you still study in that school.

I don’t know whether (if) he likes that film.

1) 引导主语从句时。例如：

Whether he will come to the party is unknown.

2) 引导表语从句时。例如：

The question is whether I can pass the exam.

3) 在不定式前。例如：

I haven’t made up my mind whether to go there or not.

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