先行词在定语从句做主语，定语从句谓语单复数由先行词决定。这里，根据先行词类别，分别举例如下:
1、 先行词为人→关系代词用who， 不能省略
The boys that are playing football are from America.
People who create computer viruses are called hackers.
The lady who stands behind the counter has bought a book.
The man who I served was wearing a hat.
2、先行词为物→关系代词用that或which，不能省略。
The books which are on the counter are theirs.
The cars which are produced in Hubei province sell very well.
A dictionary is a book which gives the meaning of words.
特别提醒:非限制性定语从句， 关系代词不能用that，只能用which。
