as 引导定语从句可置主句之前、中间或之后，而which引导的定语从句只能置于主句之后，这正是as和which引导的非限制性定语从句的区别之所在。
Such instruments as computers and printers can be found in any college.
As I mentioned earlier， most of the new features in Internet Explorer 5 aredesigned for Web developer.
A magnetic field may be represented in the same manner as an electric field.
七、主语从句
用作主语的句子叫做主语从句。引导主语从句的联系词有连词 that、whether，只起连接作用，在从句中不充当句子成分，that无词义；连接代词what、whatever、which、who、whoever，既起连接作用，又在句子中充当句子成分，有词义；疑问副词how、where、wherever、why等，既起连接作用，也在句子中充当句子成分，有词义。
主语从句的位置：
（1）放置句首
联系词引导的主语从句＋主语的谓语动词＋其他成分。如：
Exactly where the navigation buttons mentioned above depends on yourbrowser.
（译文：前面所提到的导航按钮的确切位置在哪儿取决于浏览器的种类。）
（2）放置句末
在科技文章中，经常用it作形式主语，而将真正的主语从句放到句子的最后面，因为主句的谓语往往比较简短。例如：
It is likely that the fifth generation of computers will replace human beings insome area in the near future.
（译文：在不远的将来计算机在某些领域很可能代替人类。）
常见的类似结构如下：
1）It is a pity （a shame， a fact， an honor， a wonder， a good thing， no wonder， etc.） that …
2）It is strange （natural， surprising， obvious， true， fortunate， wonderful， possible， likely， clear， unusual， etc.） that …
3）It seems （happened， thus came about， turned out， suddenly struck me， occurred to me， etc.） that …
4）It was said （reported， announced， arranged， decided， expected， etc.） that …
八、that的用法
1．指示代词 that 除了充当名词的前置定语外，还可以用来指代所论及的人或事物以避免重复，有时代替上文的整个句子，充当下句的主语。如：
Computers are of great help to our technicians. That is why we widely use them in different branches of science and engineering.
2．that充当连词，引导名词从句。只起连接作用，在从句中没有实际词义。在由that引起的宾语从句中。
I said that I would go abroad.
3．that用于加强语气的It is …that …的句型中。
It is the right time that we should go to school.
4．that 引导状语从句
（1）but that引导非真实性条件句
But that they supported us， we should have failed.
（2）in that 引导原因状语从句
In that he is the manager， he would preside the meeting.
（3）now that 引导原因状语从句
Now that we have finished the whole book， let’s review our lesson.
（4）or the reason that …引导原因状语从句（实际上that引出同位语从句，补充说明reason的内容）。
I eat potatoes for the ****** reason that I like them.
（5）with the result that …引导结果状语从句
I was in the bath， with the result that I didn’t hear the telephone.
（6）only that 引导目的状语从句、非正式条件句
He usually praises others only that he may be praised.
She would help you， only that he is too busy.
（7）so … that …引导结果状语从句
He is so tired that go to sleep as soon as he get home.
（8）so that …引导目的状语从句
Mary studies English hard so that she could go abroad.
5．that 充当关系代词，引导限制性定语从句。若 that 在从句中充当宾语时，可以省略。
但是如果先行词在如下几种情况之一：在先行词之前有same、all；先行词是who、whom或which；先行词是两个或两个以上的名词；先行词前有数词、形容词最高级作前置定语时，只能用that 引导定语从句。
Theory is valuable in that it can provide a direction for practice.
They discussed the matter in details， with the result that they came to anunderstanding.
I came to see you on the ground that Mr. Anderson said that you wereinterested in our project.
九、句型
按照一定的语法结构排列的若干词能够表达一个完整的意思的语言单位叫做句子。根据使用目的，句子可以分为陈述句（Statement Sentence）、疑问句（Question Sentence）、祈使句（Command Sentence）和感叹句（Exclamation Sentence）。
陈述句用来叙述事实，有肯定陈述句（Affirmative statement）和否定陈述句（Negative Statement）。
疑问句用来提出问题。常见的疑问句有五种：一般疑问句（ General Questions），也称为是非疑问句；特殊疑问句（Special Questions）；选择疑问句（Alternative Questions）；反意疑问句（Disjunctive Questions）和反问句（Rhetorical Questions）。
反问问句尽管在结构上与一般问句相同，但一般用升调；在形式上是疑问句，但一般不需要回答。就作用而言，一个肯定式的反问问句相当于一个语气很强的否定陈述句；一个否定式的反问问句，相当于一个语气很强的肯定陈述句，所以称之为修辞性疑问句。例如：
（1）Can you input the data without a keyboard？
没有键盘你能输入这些数据吗？
（2）Didn’t I tell you that you could easily find the information through your computer on your desk？
难道我没有告诉你，通过桌上的计算机你可以轻而易举地找到所需要的信息？
（3）The computer is really useful for us all. Who doesn’t know that？
计算机对我们大家真有用，谁不知道这个？