which前面的先行词是物,whowhom前面的先行词是人,who在从句中作主语,whom做宾语或介词宾语；that前面的先行词既可以是物也可以是人.thatwhichwho在从句中作主语时不能省,做宾语时可以省.Themanwhorobbedherhasbeenarrested.抢她东西的人被捕了.

　　ThemanwhomIsawtoldmetowait,我见到的那个人让我等一会.同时这里的whom也可以省略,也可以用that代替.

　　Thedogwhich/thatwaslosthasbeenfound.丢失的狗找着了.这里which/that作主语不可省.

　　Thecar（which/that）Ihiredbrokedown,我租的汽车坏了.thatwhich可省.