ii. as作连词引导原因状语从句，与because的区别

1. 一般说来，as表示听者已经知道的原因，因而常常位于句首；而because表示听者未知的原因，因而常常位于句尾，成为信息的焦点。

I don't want it because it is too long .

我不想要它，因为它太长了。

As I am leaving tomorrow, I've bought you a present.

因为明天我要走，我给你买了件礼物。

As all the seats were full, we had to stand on the bus.

由于座位全满了，我们只好在公共汽车上站着。

It was easy for me to guess what he was going to do as he was my close friend.

我很容易猜出来他要干什么，因为他是我的好朋友。

2. as从句不能回答why提出的问题，而because从句可以回答why提出的问题。

—Why shouldn't I come?

—Because you're too busy.

——为什么我不应该来？

——因为你太忙了。

3. as从句不能在强调句型中作为被强调的部分，而because从句能在强调句型中成为被强调的部分。

It was because he was ill that he missed yesterday's class.

正是因为生病了，他才误了昨天的课程。

专项练习

1. _______ I am ill, I can't go.

A. As

B. Since

C. Because

D. For

2. She lied to us _______ she had to do in order to get a job.

A. as

B. since

C. because

D. for

3. _______ the weather was fine, we decided to walk to the restaurant.

A. As

B. Since

C. because

D. For

4. It was _______ he missed the bus that he came late.

A. as

B. because

C. for

D. since

5. —Why did you change your mind?

—_______ I found the plan was unreasonable.

A. As

B. For

C. Since

D. Because

答案速查　1-5　ACABD