as 的基本意思：

prep. 當作，作為

conj. 正如，因為，像，按照，雖然

adv. 一樣，同樣

pron. 這一點，凡…的

1. 用作連詞，表示讓步 (意為：雖然，盡管)，要用于倒裝句，且倒裝后位于句首的名詞通常不用冠詞 (等于though, 但語氣稍弱)：

Child as [though] he was, he did quite well. 他雖是個孩子，但已干得很不錯。

2. 用作關系代詞，主要用法有二：

(1) 用在such, same, as等之后，引導限制性定語從句：

This is the same watch as I lost. 這塊表跟我丟失的那塊一樣。

Such men as (=Those men who) heard of him praised him. 聽說過他的人都贊揚他。

(2) 單獨用作關系代詞，引導非限制性定語從句，可放在主句之前 (常譯為：正如)或之后 (常譯為：這一點)，且主從句之間一般要用逗號隔開：

He was absent, as is often the case. 他缺席了，這是常有的事。

As was expected, he succeeded at last. 正如我們所料，他終于成功了。

3. 用來表示目的，下面兩種句子結構都可以：

仔細寫以便把每句話都寫清楚。

正：Write carefully so as to make every sentence clear.

正：Write so carefully as to make every sentence clear.

4. 在正式文體 (尤其是文學體裁)中，as后可用倒裝語序表示“…也一樣”這類意思 (現代英語通常so表示這一用法)：

He travelled a great deal, as did most of his friends. 他去過許多地方旅游，他的多數朋友也是一樣。

He plays the piano, as does his mother. 他會彈鋼琴，他母親也會。

注：用作連詞，表示原因 (參見because)，表示時間 (參見when)。

5. 用于 as if / as though, 意為“仿佛”、“似乎”。兩者一般可通用 (但注意不能說 as although)。用法上注意幾點：

(1) as if [though] 可引導狀語從句和表語從句，從句謂語根據語義的要求，可用陳述語氣 (若可能為事實)或虛擬語氣 (若為假設或不太可能是事實)：

It looks as if it is going to rain. 天似乎要下雨了。

It seems as if you’re right. 似乎你是對的。

He talks as though he knew everything. 他夸夸其談，好像無所不知。

When a pencil is partly in a glass of water, it looks as if it were broken. 當把一支鉛筆的一部分放在一杯水里時，它看起來好像折斷了似的。

當主句謂語是過去時態時，從句謂語常可用陳述語氣：

He paused as though he found a difficulty. 他停了下來，似乎遇到了一個難題。

She felt as if she lost something. 他覺得似乎丟了什么東西。

(2) 當從句主語與主句主語一致，且從句謂語中又包括有動詞be時，從句主語及其謂語中的動詞be通常可以省去：

From time to time he turned round as though (he was) searching for someone. 他不時地四下張望，好像在找人。

He opened his mouth as if (he was) to speak. 他張開口，好像要說話似的。

6. 關于 as…as, 用法注意：

(1) 中間只能用形容詞或副詞的原級，不能用比較級或最高級。如果在第二個as之后使用獨立的人稱代詞，一般說來用主格 (較正式)或賓格 (非正式)均可：

He’s as old as she her. 他年紀跟她一樣大。

注意：要是第二個as之后的人稱代詞跟有謂語動詞，則該人稱代詞只能用主格：

He is as old as she is.

但是有時用主格或賓格含義差別很大：

You love him as much as I (=as I love him). 你愛他像我愛他一樣。

You love him as much as me (=as you love me). 你愛他像愛我一樣。

(2) 在肯定句中用as…as, 在否定句中用not as…as或not so…as 均可：

He doesn’t study as [so] hard as his brother. 他學習不如他弟弟努力。

(3) 要注意比較對象的一致性，即比較的雙方必須是可以比較的的同類人或物，否則會出錯：

正：The population of China is much larger than that of Japan. 中國人口比日本人口多得多。

誤：The population of China is much larger than Japan.

(此句錯在將 the population of China 與 Japan 來比較，比較對象不一致)

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