as 的用法

1. 用作介词，主要表示“作为”“当作”“充当”“如同”等。如：

He treats me as a child. 他把我当小孩看待。

As your doctor, I advise you to eat less. 作为你的医生，我建议你少吃点。

另外，它还可表示“当……时候”“例如”。如：

As a child he was sent to six different schools. 他儿时前后上过6所学校。

I was wearing clothes for dirty work, as when gardening. 我穿着做粗活的衣服，例如栽花种草时所穿的衣服。

2. 用作连词，注意以下用法：

(1) 表示“随着”，后接句子。如：

As time passed, things seemed to get worse. 随着时间的推移，情况似乎变得更糟了。

若其后不接从句，而接名词，则用介词 with 表示“随着”。如：

With the development of modern agriculture and industry, more and more waste is produced. 随着现代工农业的发展，污染越来越严重了。

(2) 表示让步，意为“虽然”“尽管”，要用于倒装句（相当于 though，但语气稍弱）。如：

Boy as he was, he behaved like a girl. 他虽然是个男孩，但举止却像个女孩。

Much as I like you, I couldn’t live with you. 我尽管很喜欢你，却不能和你一起生活。

Try as he would [might]，he couldn’t open the door. 他试过多次了，却仍打不开门。

该用法请参见although词条。

(3) 表示时间，意为“当……时候”。如：

He dropped the glass as he stood up. 他站起来时，把杯子摔了。

As I was coming here, I met your brother. 我来这里时碰到了你的弟弟。

注意，as引导时态状语从句时，其谓语动词通常只能是动作动词，而不能是静态或状态动词。如：

她出生后不久父母双亡，是由姑母抚养大的。

误：Her parents died as she was a baby and she was brought up by her aunt.

正：Her parents died when she was a baby and she was brought up by her aunt.

但是，当as用于引导原因状语从句时，从句谓语可以是静态动词或状态动词。如：

As you weren’t there I left a message. 因为你不在那里, 我留了个信儿。

3. 用作关系代词，主要用法有：

(1) 用在 such, same, as 等之后，引导限制性定语从句。如：

I live in the same place as Tom does. 我和汤姆住在同处。

He’s not such a fool as he looks. 他并不像看上去的那样愚蠢。

He smokes as expensive a cigarette as he can afford. 他抽一种他买得起的最贵的烟。

(2) 单独用作关系代词，引导非限制性定语从句，可放在主句之前、之后或中间，且主从句之间一般要用逗号隔开。如：

As was expected, he succeeded at last. 正如我们所料，他终于成功了。

I had coffee after dinner, as is my usual practice. 我饭后喝咖啡，这是我的习惯。

Freddie, as might be expected, was attending the conference. 弗雷迪在参加会议，这是可以预料到的。

(3) as与which的区别

① 引导限制性定语从句的区别：as用作关系代词引导限制性定语从句时，它通常只用于 such, same, as 等之后，而which 用作关系代词引导限制性定语从句时，则用于除 such, same, as 之外的其他结构。

② 引导非限制性定语从句的区别：which引导非限制性定语从句时，通常只能位于主句后，而as引导的非限制性定语从句位于主句之前、之后或中间。

根据从句谓语动词的特点来看，as在定语从句中作主语时，从句谓语通常要有be或别的系动词，但which不受此限制。如：

Li Ming was late, which (=and this) made Mr. Zhang very angry. 李明迟到了，这件事使张先生非常生气。（不可用as)

根据句意的逻辑性来看，as引导的定语从句只表示一个众所周知或意料之中的事，但which不受此限制。如：

He married her, as [which] was natural. 他和她结婚了，这是很自然的。

He married her, which was unexpected. 他和她结婚了，这是出乎意料的。（不可用as）