which 在定语从句中的用法
关系代词 which 一般指物，在定语从句中作主语、宾语或定语，既可引导限制性定语从句，又可引导非限制性定语从句。下面就进行归纳总结：
1. 引导限制性定语从句
A shop should keep a stock of those goods which sell best.
商店应存有最畅销的货物。
This is the family which is planning to move to the party.
这是要搬进城里的一家。
2. 引导非限制性定语从句。
（ 1 ）用来指代一个句子。
Internet is so interesting, which makes all possible happen.
因特网是如此有趣，它使任何事都有可能发生。
（ 2 ）用来指代句子的一部分。
When deep in thought, which he often was, he would forget all around him.
他常常陷入沉思，这时他就会忘掉周围的一切。
（ 3 ）如果要引导两个非限制性定语从句，第二个 which 前要加 and.
He bought a book, which was written by LuXun, and which he decided to give to his friend.
他买了本鲁迅写的书，他决定送给朋友。
3. 名词＋ of ＋ which （＝ of which ＋名词＝ whose + 名词）通常放在先行词的后面。
I'd like a room the window of which looks out over the sea. / I'd like a room of which the window looks out over the sea. / I'd like a room whose window looks out over the sea.
我要一间窗户面临大海的房间。
4. 介词＋ which 的替代作用。
（ 1 ）作时间状语替代 when 。
There used to be a time at which / during which （ = when ） the Chinese people struggled for freedom.
中国人曾有一段为自由而斗争的日子。
（ 2 ）作地点状语替代 where 。
This is the office in which （ = where ） I used to work.
这是我过去工作过的办公室。
（ 3 ）作原因状语替代 why 。
I'd like you to explain the reason for which （＝ why ） you were absent.
我想让你解释一下缺席的原因。
（ 4 ）作方式状语替代 that 或省略。
There are many ways in which （＝ that / 省略） we can solve the problem.
解决这个问题有很多方法。
5. which, when, where 关系词的选择。
关系副词 when 与 where 用于引导定语从句时，往往修饰句中表示地点或时间的名词。但是，有时候在表示时间或地点的名词后面却不能用 when 或 where ，而该用 which 或 that 。
Is this the factory which / that produces all kinds of washing machines? （ which / that 在定语从句中作主语，指代前面的 the factory ）这就是生产各种洗衣机的那家工厂吗？
I often think of the days which / that we spent together on the seaside. （ which / that 在定语从句中作宾语，指代 the days ，此时可省略。）我经常想起我们一起在海滩度过的日子。
The river where I used to swim is now seriously polluted.
（ where 在定语从句中作状语， where = in which = in the river ）我过去常去游泳的那条河现在已被严重地污染了。
I still remember the day when I first came to Beijing.
（ when 在定语从句中作状语， when = on which = on the day ）我仍然记得我第一次来北京的日子。
总结：在表示时间或地点的名词后面，选择何种关系词，应看它们在从句中的作用，即在从句中充当什么成份。如果在定语从句中充当时间或地点状语，就该用 when 或 where ；如果在定语从句中充当主语或宾语等，则应用 which / that 。
6. 引导非限制性定语从句关系代词 which, as 的选择。
关系代词 as 与 which 引导非限制性定语从句时可以用整个句子作为先行词，指代上文或下文所说的一件事。
He said he'd been working in the office for an hour, which / as was true.
他说他一直在办公室工作了一个小时，这是真的。
总结：
（ 1 ） Which 指前面主句所提到的这件事，常译为“这件事，这一点”等； as 具有“正如、像、由……而知、与……一致”的意思。
He was elected mayor of the city, which made us happy.
他被选为这个市的市长，这使我们很高兴。
As we expected, he didn't appear at the party.
正如我们所预料，他没有在聚会上出现。
（ 2 ） Which 在句法上一般用作实义动词的主语，这时它所引导的从句与主句之间常含有并列、因果关系；句法上， as 常作一些实义动词（如 see, know, report, watch, remember, say, tell, show, expert, guess 等）的宾语。
I bought my sister a big toy, which （ = and that ） delighted her greatly.
我给妹妹买了一个大玩具，这使她非常高兴。
As is known to everybody, the moon travels round the earth once every month.
众所周知，月球每个月环绕地球运行一次。
（ 3 ） which 引导的非限定性定语从句位置比较固定，通常位于句末，不可移至句首；而 as 所引导的非限定性定语从句的位置比较灵活，可以位于句末，也可以位于句首或句中。
Taiwan is a beautiful island, as （＝ which ） we know.
正如我们所知，台湾是一个美丽的岛屿。
As （不用 Which ） he realized, I was very useful to him.
他意识到我对他很有用。
Mary, as we had expected, passed the exam.
正如我们所预料， Mary 通过了考试。
（ 4 ）当从句的谓语动词是否定形式或从句谓语动词接了一个复合宾语结构时，一般用 which 而不用 as 。
He pretended not to know me, which I didn't understand.
他假装不认识我，我真不明白为什么。
He admires everyone in the class, which I find quite strange.
他羡慕班里的每个学生，我觉得很奇怪。
（ 5 ）不能省去非限定性定语从句中的 be 动词时，用 which （反之则用 as ）。
Jane told me she won the match, which was a lie.
简告诉我她赢得了比赛，这是谎话。（ was 不可省略）
As (was)planned, we met at the airport.
正如计划那样，我们在飞机场见面了。（ was 可省略）
ic的种类总共有多少种