3
、
在一个句子中有两个定语从句，
其中一个定语从句的关系代词用了
that,
另一个宜用
which
。
例如：
Let me show you the novel that I borrowed from the library which was newly open to us.
我来给你看我从新开的图书馆里借来的那本小说。
4
、当关系代词后面有插入语时。例如：
Here is the English grammar which, as I have told you, will help improve your English.
这就是我跟你说过的会有助于你学好英语的那本英语语法书。
5
、先行词为
that
时。例如：
The clock is that which can tell us the time.
钟是报时的装置。