英语里的 as 是一个比较常见、用法繁多的词。它和其它词的搭配关系也很广泛。现归纳为以下几种作用。至于它的特殊用法, 即在各种固定词组里的用法, 我们打算在下一个专题讨论（见本节后面 As 在各种句型和固定词组里的用法）。
一、as 用作关系代词, 引出定语从句
I. such … as 像 … 这（那）样的, 像 … 之类的。as 和 such 连用, such 在下列句子里作定语用, 说明主句里的某个名词。as 所引导的定语从句修饰前面的名词, 而 as 本身在从句里, 可能是主语、宾语或表语。
We Will give you such data as will help you in your work. 我们要提供给你们那种对你们工作有帮助的资料。（as 作从句里的主语。）
He is not such a man as would leave his work half done. 他并不是一个做事半途而废的那种人。（as 作从句里的主语, half done 作宾语补语。）
I hope to get such a tool as he repaired this instrument with. 我希望能够得到一个像他用来修理这个仪器的那类工具。（as 作 with 的宾语, 这个介词短语作状语。）
II. the same … as 和 … 同样的
This is the same tool as I used yesterday. 这和我昨天所使用的工具是相同的。（as 作从句的宾语）
I should like to use the same instrument as is used in your workshop. 我想用你们车间里所用的同样的仪器。（as 作从句的主语。）
He is not the same man as he was. 他和过去不同了。（as 作从句的表语。）
Molecular weights are expressed in the same units as atomic weights. 分子量以原子量所用的相同单位表示之。（as 作关系副词, 定语从句内省略了与主句相同的谓语 are expressed。）
If one object is charged with the same kind of electricity as appears on another nearby, the two objects will repel each other. 如果一个物体带有与它邻近的物体上所出现的同种类的电荷, 这两个物体会相互排斥。（as 作从句的主语。）
This is the same instrument as we stand in need of. 这正是我们迫切需要的那种仪表。（as 作介词 of 的宾语。）
The Chinese people are not the same as they were before. 中国人民跟以前不同了。
Radio waves travel at the same speed as light in empty space or in air. 在空洞洞的宇宙和空气申, 无线电波的传播速度和光速相同。
They do the same job as we do. 他们*是和我们相同的工作。
Heat radiation is of the same nature as light, X-rays and radio waves. 热辐射就其性质而言, 是和光、X光及无线电波相同的。
注：注意“… same as”有时发展到接近于 just as 引出方式状语从句的用法。例如：
She has put her energy into literature — same as you have put yours into computers. 正如你把全部精力投入计算机那样, 她也把全部精力投入到文学了。
III. as many … as, as much … as（分别用于可数和不可数名词）如 … 一般多；如此之多；凡 … 的 … 都
As many instruments as are in the laboratory have been made most use of. 实验室里那么多的仪器都已充分利用了。（as 作从句的主语）
There are as many books as are needed. 凡是需要的书都有了。（as 作从句的主语）
He has as many books on information technology as I have. 他拥有的信息技术书和我拥有的同样多。（as 在从句中作宾语）
Here is a jar of distilled water. You may use as much as you need. 这里有一大瓶蒸馏水, 你需要多少就可以用多少。（as 在从句中作宾语）
These generators supply only half as much energy as required. 这些发电机只供应所需能量的一半。（第二个 as 在从句中作主语, 其后省略了 is。）
下句中可把 as（指第二个 as）理解为引出相比较的具体内容：
He is a man to whom work means as much as life. 他是一个把工作看作和生命一样重要的人。（to whom… 也为定语从句。）
As much as half a foot of snow has fallen in Tianjin. 天津已下了半英尺厚的雪。
At this weather analysis center, as many as 35 different charts were constructed for a given time. 当时在这个气象分析中心要为同一个规定的时间绘制多达 35 种不同的图表。
