(1)不用that的情况
(a)在引导非限定性定语从句时。
(错)Thetree,thatisfourhundredyearsold,Isveryfamoushere.
(b)介词后不能用。
Wedependonthelandfromwhichwegetourfood.
Wedependonthelandthat/whichwegetourfoodfrom.
(2)只能用that作为定语从句的关系代词的情况
(a)在therebe句型中,只用that,不用which。
(b)在不定代词,如:anything,nothing,theone,all等作先行词时,只用that,不用which。
(c)先行词有theonly,thevery,thesame,thelast,just修饰时,只用that。
(d)先行词为序数词、数词、形容词最高级时,只用that,不能用which。.
(e)先行词既有人,又有物时。
(f)先行词指物,在主句中作表语时.
(g)为了避免重复.
(h)先行词是theway或thereason时,that可作关系副词,也可省略
(i)主句的主语是疑问词who/which时