先行词是人,引导词可用who或that
先行词为物,可用which或that
whom在定语从句中做名词定语
通常使用that：
1.当先行词是all,anything,everything,nothing,little,much,none,one时,大都使用that或省略作宾语的that（即零关系代词）
Is there anything else that you want me to get?
你还有别的东西要我去买吗?
2.当先行词有only,all,any,few,little ,no,one of等修饰语时,通常用that或省略作宾语that.
The only thing that impresses me is her directness.
唯一给我留下深刻印象的就是她的直率.
3.当先行词有序数词、形容词最高级、last、next修饰时,通常用that或省略作宾语的that.They eat the finest food that is available.
他们吃所能买到的最好的食品.
4.当先行词既含有表人的名词又含有表物的名词,只能使用that.
通常使用which：
1.当关系代词前有介词时,只能使用which.
Here is the car about which I told you.这就是我向你谈到的那辆车.
2.当关系代词之后有插入成分时：
I have received your letter which,as I have told you,was needed by the press.
我已经收到你的来信,这封信,正如我所告诉你的,是新闻界所需要的.
