1. 用作介詞，意為“作為”、“當(dāng)作”、“充當(dāng)”。如：

He treats me as a child. 他把我當(dāng)小孩看待。

As your doctor, I advise you to eat less. 作為你的醫(yī)生，我建議你少吃點(diǎn)。

2. 用作連詞，注意以下用法：

(1) 表示“隨著”。如：

As time passed, things seemed to get worse. 隨著時(shí)間的推移，情況似乎變得更糟了。

若其后不接從句，而接名詞，則用介詞 with 表示“隨著”。如：

With the development of modern agriculture and industry,

more and more waste is produced. 隨著現(xiàn)代工農(nóng)業(yè)的發(fā)展，污染越來(lái)越嚴(yán)重了。

(2) 表示讓步(意為：雖然，盡管)，要用于倒裝句(相當(dāng)于 though, 但語(yǔ)氣稍弱)。如：

Much as I like you, I couldn’t live with you. 我盡管很喜歡你，卻不能和你一起生活。

Try as he would [might]，he couldn’t open the door. 他試過(guò)多次了，卻仍打不開(kāi)門。

注：倒裝后位于句首的名詞通常不用冠詞。如：

Boy as he was, he behaved like a girl. 他雖然是個(gè)男孩，但舉止卻像個(gè)女孩。

3. 用作關(guān)系代詞，主要用法有：

(1) 用在 such, same, as 等之后，引導(dǎo)限制性定語(yǔ)從句。如：

I live in the same place as Tom does. 我和湯姆住在同處。

Such people as knew him admired him. 認(rèn)識(shí)他的人都?xì)J佩他。

He smokes as expensive a cigarette as he can afford. 他抽一種他買得起的最貴的煙。

(2) 單獨(dú)用作關(guān)系代詞，引導(dǎo)非限制性定語(yǔ)從句，可放在主句之前(常譯為：正如)或之后(常譯為：這一點(diǎn))，且主從句之間一般要用逗號(hào)隔開(kāi)。如：

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。