什么情况下定语从句必须用that

定语从句中关系词必须用that的情况：
1. 被修饰的先行词是不定代词all,much,something,everything,nothing,none,the one等时,（注意：表示人的不定代词即可用that也可用who/whom）
如：Anyone who/that doesn”t like this film must tell me.
Do you have something that is very difficult to understand?
All（that）I can do is to give him some hope.
2. 被修饰的先行词还有一些特定意义的修饰词,如：the only、the very、just the、the right、any、every、no、all、much、many、little、few等,
如：The only thing（that）we know is that the universe is very big.
Any bus that can go there is OK.
This is just the/the very/the right book I am looking for.
There are few cars that are driving slowly.
Every car that has a high price is worth what it costs.
3. 被修饰的先行词还有序数词或最高级修饰,
如：This is the fifth book（that）you have read.
This is the best book（that）I have ever read.
4. 先行词既有人又有物或time,
如：Can you tell me the people and events（that）you saw in Britain?
The man and his dog that looked odd came to me quietly.
Each time that he came here, he would like to talk with her.
5. 句子中出现过which或who之类的词,为了避免重复,要用that,
如：Who is the man that is speaking?
The factory which produces shoes that are very good has gone bankrupt.

1、先行词有人又有物或时间，必须用that。
2、当先行词是不定代词或被不定代词修饰的时候，用that。
3、当先行词被最高级，序数词，the very,the right,just the,the last ,the only 等修饰时，用that。
4、当句子中出现过which 或who之类的词，后面的关系词只能用that。...