这样理解完全正确.
定语从句中关系代词用that(不用which)的情况:
1.先行词是all,anything,everything,nothing等,关系代词应用that
That'sallthathetoldme.
2.先行词前有theonly,thesame,thevery或no,little,much,any,every,all等修饰时,关系代词应用that
SheistheonlypersonthatIknowinthiscompany.
3.先行词前有序数词或形容词最高级修饰时,关系代词常用that
ThefirstthingthatIwanttodoistotakeitaway.
4.当先行词同时含有"人"或"物"时,关系代词应用that
5.在固定结构thesame...that...;so...that...;such...that...以及itis/was...that...的强调句型中,要用that
Thisisthesameparkthatweoncevisited.
6.如主句以therebe开头时,关系代词应用that
Thereisaroomthathastwowindows.
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