as和which引导非限制性定语从句的区别
（1）位置的不同
Which（放句中） 引导的定语从句只置于所限制的句子后；as 位置较灵活，也就是说as可置于所限制的句子前面；插在句子中或放在句子后。如：
He was late again, which made his teacher very angry.
他又迟到了，这使他的老师很生气。
Jack, as you know, is an honest man.
或Jack is an honest man , as you know.
（2）先行词的不同
as引导非限制性定语从句时，其先行词多为一个句子；which引导非限制性定语从句时，其先行词可以是一个词，一个短语或一个句子。
He was proud, which his brother never was. （先行词是一个词）
他感到骄傲，他的兄弟从来没有。
She was very patient towards the children, which her husband seldom was.
她对孩子很有耐心，而她的丈夫很少。
He was proud, which I dislike very much.（先行词是一个句子）他很骄傲，这是我很讨厌的地方。
（3）as 一般译为"正如""就像"，"这一点"
as we all know；as you know; as is known to all; as you see; as we can see; as has been expected; as we have imagined.
who & that
一个句子中带有两个定语从句，其中一个定语从句的关系代词是that ,另一个则宜用who, 以免重复。如：
The student that was praised at yesterday's meeting is the monitor who is very modest and works very hard .
昨天在会上受到表扬的那个学生是班长，他很谦虚而且工作很努力。
【小试牛刀】
1．The sides of the Iran nuclear talks reached a key point, the result of ________ would have a great influence on the world peace.
A．that B．which C．whom D．them
2．The famous scientist, ________ a dinner party will be held tonight, is to arrive soon.
A．in his honour B．in whose honour C．in which honour D．in honour of him
3．—So when did you meet John?
—It was last Sunday ________ he came to visit my sister.
A．that B．since C．when D．before
4． In our class there are 46 students, ________half wear glasses.
A．in whom B．in them C．of whom D．of them
5． Alex has finally become an astronaut, ________all his family members think is his dream career.
A．which B．who C．that D．whom
【答案与解析】
3. C 【解析】考查定语从句。句意：“那么你是什么时候见过约翰呢？”“是上周日他来看我妹妹的时候。”答句是一个省略的强调句型，省略了that I met John，强调时间状语last Sunday，且强调句型中包含了一个when引导的定语从句，故选C项。
4. C 【解析】考查定语从句。句意：我们班有46个学生，其中一半戴眼镜。先行词为students，所以用of whom表示部分与整体的关系，故选C项。
5. A 【解析】考查定语从句。句意：亚历克斯终于成为了一名宇航员，他所有的家人认为这是他的梦想职业。先行词an astronaut表示职业，所以用which引导非限制性定语从句，故选A项。
