1、as引導時間狀語從句，意為“正當……的時候”、“一面……一面……”、“隨着……”，從句可放主句之前或主句之後。
As I was coming here, I met your brother.
As he walked on, he felt himself getting more and more tired.
【知識拓展】：as與when, while都是引導時間狀語從句的從屬連詞，含義都是“當……的時候”。但它們有區別：用when時，從句的動作可以與主句的動作同時發生，也可以先於主句的動作發生；用while時，從句的動作為一過程，主句動作與從句動作同時進行或在從句動作過程中發生；用as時，主句和從句的動作同時發生，具有延續的含義。例如：
It was snowing when he arrived at the construction site.他到達工地時，天正在下雪。
He was watching TV while his mother was cooking.他母親做飯時他在看電視。
You will grow wiser as you grow older.你會隨着年齡的增長而越來越聰明。
2、as引導原因狀語從句，意為“由於”大多位於主句之前，偶爾在前或後。
As this question is of great importance, we will discuss it once again.
Water, as it occurs in nature, is never very pure.
We couldn‘t send the children to school as life was very hard in those days.
【知識拓展】：as, because, since都可以表示因果關係，連接原因狀語從句，含義是“因為，由於”，但它們有區別：because表示的語氣最強；as一般放在句首，語氣較弱，較口語化；since常常用在書面語中，表示多為對方已知的、或稍加分析便可得知的原因，有時可譯作“既然”。例如：
He will succeed because he is in earnest.他一定會成功，因為他很認真。
Since you are so sure of it he’ll believe you.既然你對此如此有把握，他會相信你的。
As rain has fallen, the air is cooler.因為下過雨，空氣比較清爽。
3、方式狀語從句，意為“如同”、“按照……的方式、樣子、辦法”。As從句總跟在主句之後。
You must try to hold the tool as I do. 象我那樣拿住工具。
Leave things as they are. 讓那些東西保持原樣。
3、as引導讓步狀語從句，意為“雖然”，多在主句之前，且多用倒裝。(應將從句主要成分的形容詞、副詞、原形動詞、不加冠詞的單數名詞置於as 前面)
Difficult as it is, we should not give it up.
Child as he is, he knows something about electricity.
【知識拓展】：as與although (或though)，however (或no matter how)等都可以引導讓步狀語從句，含義是“雖然，儘管”，但它們有區別：although語氣稍正式些，可放在句首，也可放在句中，主句中不能再用but，但可以用yet;as所表示的語氣較強，引導的讓步狀語從句用倒裝語序；however引導讓步狀語從句時，它的後面可跟形容詞或副詞，也要用倒裝語序。例如：
Although he is quite old, he still jogs every day.他雖然年紀相當大，仍然每天慢跑。
Strange as it may seem, nobody was injured in the accident.
這次意外雖然顯得令人不可思議，卻沒有人受傷。
However hard he may try, he will not attain his goal.無論他怎樣努力也達不到目標。
4、as引導比較狀語從句，意為“和……一樣/不一樣”。 表示同級的比較。使用時要注意第一個as為副詞(意為“那麼”“如此”)，第二個as為連詞(引導的比較狀語從句常常用省略)。其基本結構為：as+ adj./ adv. +as。例如：The line AB is as long as the line CD (is).
This film is as interesting as that one.這部電影和那部電影一樣有趣。
Your pen writes as smoothly as mine.你的鋼筆書寫起來和我的一樣流暢。
注意1：其否定式為not as/so +adj./ adv. +as。例如：
This dictionary is not as/so useful as you think.這本字典不如你想象的那樣有用。
注意2：“as…as…”結構前加上表示程度或倍數的詞語作為修飾成分，如just, nearly, almost, twice, three times, half, a quarter等，則須置於第一個as之前。例如：
Your bag is twice as expensive as mine.你的袋子比我的貴一倍。
This is just as important an experiment as that(is).
There are now three times as many schools in this city as(there were )ten years ago.
【知識拓展1】：幾個關於的常見句型：
(1) possible
Please answer my question as soon as possible.請儘快回答我的問題。
(2) usual/before
She looks as pretty as before.她看起來和以前一樣漂亮。
(3)as long as... (引導條件狀語從句)
It took us as long as three years to carry out the plan.我們花了長達三年的時間才完成這項計劃。
1、引導限制性定從句。
①用於the 結構中。
This is the same book as I read last week.這本書和我上週讀的那本是一樣的。
He isn’t the same person as he used to be.
②用於結構中。
I have never heard such story as he is telling.
I don’t like such books as he recommends. 我不喜歡他推薦的那些書。
③用於“as/so +adj. + a/an + n. (單數) + as ”結構中
I am not so strong a man as I was.我已經沒有從前那麼強壯了。
2、引導非限制性定從句，修飾整個主句或主句的一部分，可在句首、句中、句末。當 as 在從句中做主語時，從句的謂語動詞常是被動語態或系表結構。在從句中做賓語時，從句的謂語動詞常是情態動詞 + 動詞原形。
As is often the case, we have overfull filled the production plan.
This machine, as might be expected, has stopped operating.
He came very early this morning, as (was) usual.
She is late, as is often the case.她遲到了，這是經常的事。(先行詞是整個主句)
To shut your eyes to facts, as many of you do, is foolish.
對事實視而不見--你們好多人都是如此的愚蠢。(先行詞是不定式短語)
此外還有： as has been said before/ as may be imagined / as is well known/ as we all can see/ as often happens/ as has already been pointed out等等。
首先，我們來想一想，什麼詞能修飾行為動詞(action verb)？沒錯，是副詞。我們看例句，I always unwrap electronic devices “carefully”。
因此，如果我們想説A和B兩個同學都是很小心unwrap電子產品的人，我們可以把副詞“carefully”放到As和as之間。
例句：John unwraps electronica devices as carefully as Michael does.
因此，As…as用法2的注意事項是我們要用一個行為動詞引導As…as之間的副詞，比如，An apple “grows” as slowly as an orange does.
注：本質上在As…as詞組中，第二個as實際上是在引導一個名詞從句，但我們往往僅保留從句的主語，即名詞，而省略其他部分。看例句，The weather this summer is as bad as last year’s (weather is)。
as…as結構的幾點用法説明:
(1)as.。 possible
Please answer my question as soon as possible.請儘快回答我的問題。
(2)as.。 usual/before
She looks as pretty as before.她看起來和以前一樣漂亮。
(3)as long as.。. （引導條件狀語從句）
It took us as long as three years to carry out the plan.我們花了長達三年的時間才完成這項計劃。
(4)as far as
He walked as far as the railway station yesterday evening.昨天傍晚，他一直散步到火車站。
(5)as well as
She cooks as well as her mother does.她燒菜燒得跟她母親一樣好。
as busy as a bee像蜜蜂一樣忙碌
as easy as ABC像ABC一樣容易
as deep as a well像井一樣深
as light as a feather像羽毛一樣輕
as soft as butter像黃油一樣軟
as rich as a Jew像猶太人一樣富裕
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