按理说，关系代词与人称代词无论在语义方面还是用法方面，都有很大的区别，一般是不会发生混淆的。但是，许多同学在做题时往往是“跟着感觉走”，而忽略对句子结构的分析，同时命题者也常常抓住考生这一毛病，就此设题，结果许多考生掉入陷阱。请看下面一题：
My friend showed me round the town, ________ was very kind of him.
A. which B. that C. where D. it
对于这道题，很显然，B和C不能选，因为that不用于引导非限制性定语从句，而where是关系副词，只用作状语，不用作主语，所以这两者很容易被排除。但是，在A和D之间，该选哪一个呢？许多同学选了人称代词it，因为It was very kind of him读起来“太有感觉了”，语感“太正常了”；但是，很遗憾，选it错了。正确答案是A，即应选关系代词which。为什么呢？
这要从英语的句子结构说起。根据英语语法，如果从结构上看，英语句子可分为简单句、并列句和复合句三类（若包括并列复合句，则为四类）。
所谓简单句，就是只有一个主谓结构。如：
Our dog likes catching biscuits in its mouth. 我们的狗喜欢用嘴接饼干。
所谓并列句，就是指由并列连词连接的两个或多个主谓结构。如：
We parted, and we haven’t met since. 我们分手了，以后再也没有见面。
At first they were happy, but things soon started to go wrong. 起初他们在一起很快乐，但不久就开始出问题了。
所谓复合句，就是指包括一个主句和一个（或几个）从句的句子。如：
He answered that he knew nothing about it. 他回答说他不知情。（that引导的是宾语从句）
It is an organization which helps the elderly. 这是一个帮助老年人的机构。(which引导的是定语从句)
He’ll have four years at university after he leaves school. 他中学毕业后要上四年大学。(after引导的是时间状语从句)
John wanted to go to the party, but his wife said she was too tired. 约翰想去参加晚会，但他的妻子说她太累了。(but连接两个句句子构成并列句，而but后的句子中又是一个含有宾语从句的复合句)
有了上面的句子结构知识，我们就来分析上面这道考题为什么该用which不用it了。现在请看这两个句子：
My friend showed me round the town, which was very kind of him.
My friend showed me round the town, it was very kind of him.
第一句用which之所以是正确的，是因为which在此引导非限制性定语从句，整个句子为一个含有定语从句的复合句；而第二句用it之所以不正确，是因为这个句子不属于上面提到的任何一种句型：首先，它不是简单句，因为它有两个主谓结构；其次，它也不是并列句，因为句子间没有并列连词；第三，它也不是复合句，因为句子间没有体现出主句和从句；第四，它更不是并列复合句，因为它既不是复合句，也不是并列句。
通过这样的分析，同学们应该知道上面这个句子用it为什么是错误的了。假若要想保留人称代词it，又要使句子不错，也有另外一个办法，就是在it之前加上一个并列连词，使之成为并列句。如说成：
My friend showed me round the town, and it was very kind of him.
现在同学们应该搞清楚了吧。下面请再看一道考题：
She brought with her three friends, none of ________ I had ever met before.
A. them B. who C. whom D. these
这道题是用them还是whom？套用上面的分析，我们可以很容易地确定答案为C，而不是A。但是，如果我们把题目稍为改变一下呢？如改成：
She brought with her three friends, and none of ________ I had ever met before.
A. them B. who C. whom D. these
这道题的答案就不能是whom，而应是them了。当然，选these在语法上也是对的，只是意思上不是很通顺，相比较而言，显然不如用them好。
最后再来看一个很典型的例子：
Simon loves you, ________ is why he wants to be with you.
A. it B. that C. which D. what
由于that’s why…是同学们非常熟悉的句式，所以许多人可能会毫不犹豫地选B。又错了！最佳答案应是C。首先，由于句子间没有并列连词，所以排除A和B；又因为what不是关系代词，不能引导定语从句，所以也不能选。注意，如果在两个句子间加上并列连词and，或将其中的逗号改为破折号或分号，则可用that，如以下三句都是正确的：
Simon loves you―that’s why he wants to be with you.
Simon loves you and that’s why he wants to be with you.
Simon loves you; that’s why he wants to be with you.
【小练一下】
01. That bar on
A. that B. how C. which D. what
02. She says it’s
A. that B. how C. which D. what
03. Anyway, that evening, ______ I’ll tell you more about later, I ended up staying at Rachel’s place.
A. that B. how C. which D. what
04. Dust particles must have got into the motor, ______ is why it isn’t working properly.
A. it B. that C. which D. what
05. I’m afraid he’s more of a talker than a doer, ______ is why he never finishes anything.
A. it B. that C. which D. what
06. He asked a lot of questions, none of ______ was easy to answer.
A. which B. them C. what D. that
07. He told me that he had two girl-friends, neither of ______ knew anything about the other.
A. whom B. them C. which D. who
【答案与解析】
01. C。引导非限制性定语从句且指事物时，要用关系代词which，而不能用that；而how和what不是关系词，均不能引导定语从句，故可排除B和D。
02. C。引导非限制性定语从句且指事物时，要用关系代词which，而不能用that；而how和what不是关系词，均不能引导定语从句，故可排除B和D。
03. C。引导非限制性定语从句且指事物时，要用关系代词which，而不能用that；而how和what不是关系词，均不能引导定语从句，故可排除B和D。
04. C。由于句子间没有并列连词，所以排除A和B；又因为what不是关系代词，不能引导定语从句，所以也不能选。
05. C。由于句子间没有并列连词，所以排除A和B；又因为what不是关系代词，不能引导定语从句，所以也不能选。
06. A。由于介词后不能接关系代词that，排除D；而them和what不是关系代词，不用于引导定语从句，也被排除。none of which在此引导非限制性定语从句，其意为“所有这些问题都不”。
07. A。由于介词后不能接关系代词who，排除D；而them不是关系代词，不用于引导定语从句，也被排除；至于which，它只用于指事物，不用于指人，也被排除。neither of whom在此引导非限制性定语从句，其意为“她们两人都不”。