关系代词that：指物,有时也指人,在从句中作主语或宾语,作主语不可省略,做宾语可省略（在非正式文体中）eg:theboythathelpedmeismybrother.thebuildingthatwebuiltlastyearisverytall.

　　关系代词which:指物,不指人,比that正式,在从句中作主语或宾语,作主语不可省略,做宾语可省略（在非正式文体中）.eg:thecoat(which/that)iputonthedeskisblack.

　　关系代词whom：指人,在从句中作宾语.eg:everyonelovesthegirlwhomwecallMary.（此处亦可用who/that/省略）

　　关系代词whose：在从句中作定语,不可省.eg:everyonelovesthegirlwhosewatchisnew.

　　关系副词where：在从句中作状语.eg:mywifewantstogotothecitywherewekneweachother.(where=inwhich)

　　what不引导定语从句,所以在定语从句中不能用what代替that.因为that是关系代词而what是连接代词,相当于thethingsthat/which...,也就是说what包含that.eg:dothestudentsunderstandwhatyousaid?=dothestudentsunderstandthewordsthatyousaid?