定语从句何时必用that我就要 that的,其他不要.什么情况下定语从句必用that!

除去前两位仁兄说的情况外，还有一种情况，就是当先行词在主句中作表语，或者是关系代词在定语从句中作表语时，只能用that.

定语从句中从句缺少主语。宾语。表语用that或者which引导从句
但在下列情况中，只能用 that，不用 which：
①当先行词是 all, a lot, (a) little, few, much, none, anything, something, everything, nothing 等词时。
②当先行词被 all, any, no, much, little, few, every 等限定词所修饰时。
③当先行词是序数词或被序数词所修饰时。
④当先行词是形容词*或被形容词*所修饰，以及先行词被序数词和形容词*同时修饰时，
⑤当先行词被 the very, the only, the next, the last等所修饰时。如：
⑥当先行词为人和物的并列名词词组时。
⑦当主句是以 which 开头的特殊疑问句时。这是为了避免引导词重复。

但在下列情况中,只能用 that,不用 which：
①当先行词是 all,a lot,(a) little,few,much,none,anything,something,everything,nothing 等词时.如：
All that can be done has been done.
In this factory I saw little / much that was different from ours.
②当先行词被 all,any,no,much,little,few,every 等限定词所修饰时.如：
We heard clearly every word that he said.
③当先行词是序数词或被序数词所修饰时.如：
The first thing that should be done is to get the tickets.
When people talk about Hangzhou,the first that comes to mind is the West Lake.
④当先行词是形容词*或被形容词*所修饰,以及先行词被序数词和形容词*同时修饰时,如：
Is that the best that you can do?
That’s the most expensive hotel that we’ve ever stayed in.
This novel is the second best one that I have ever read.⑤当先行词被 the very,the only,the next,the last等所修饰时.如：
This is the very book that I want to find.
⑥当先行词为指人和指物的两个并列名词词组时.如：
The guests spoke highly of the children and their performances that they saw at the Children’s Palace.
She described in her compositions the people and places that impressed her most.
⑦当主句是以 which 开头的特殊疑问句时.如：Which is the car that killed the boy?