④定语从句出现在which或who的特殊疑问句中时用that。

　　Which is the bike that you lost?

　　哪辆是你丢的自行车？

　　Who is the boy that won the gold medal?

　　赢得金牌的那个男孩是谁？

　　⑤有两个定语从句时，其中一个关系代词宜用which，另外一个宜用that。

　　They secretly built up a small factory, which produced things that could cause pollution.

　　他们偷偷地建了一家小工厂，这家工厂生产可能会造成污染的东西。

　　⑥当先行词在主句中作表语，而关系代词在从句中作表语时。

　　Shanghai is no longer the city that it used to be.

　　上海不再是过去的那座城市了。

　　⑦主句是there be句型且关系词在从句中作主语时，用that不用which引导。

　　There is a seat in the corner that is still available.

　　在那个角落还有一个座位可用。

　　(2)只用which不用that的情况：

　　①引导非限制性定语从句。

　　Helen was much kinder to her youngest son than to the others, which, of course, made the others envy him.

　　海伦对她最小的儿子比对其他的儿子好得多，这当然让其他的儿子很嫉妒她的小儿子。

　　②关系代词充当介词的宾语，且介词位于关系代词之前。

　　This is the house in which Lu Xun once lived.

　　这是鲁迅曾住过的房子。

　　③引导的定语从句修饰前面的整个主句，代替主句所表示的整体或部分概念。

　　The result was not the same as they had expected, which was rather disappointing.

　　结果与当初预料的不一样，这让人觉得很失望。

　　2．which与as引导非限制性定语从句的区别

which as 位置上 只能放在主句的后面 位置灵活，可放在主句前也可放在主句后，有时还可插入主句中 搭配上 无动词的限制 谓语动词通常是表示感觉或心理活动的动词，如see, hear, know, expect, remember等 意思上 意为"这一点" 意为"正如......，正像......的那样" 　　

　　She was late for school again, which was unexpected.

她上学又迟到了，这是始料不及的。