定语从句中规定,如果先行词,就是被修饰的名词如果是下面情形时,必用关系词 that ：
1.先行词为人和物时
I saw a boy and his dog that were walking in the park.
2.先行词有序数词或形容词最高级时
This is the most interesting book that I have ever read.
3.先行词有the only / the very / the same + 名词
He was the only person that knew how to open the safe.
而你知道 that 如果在后面定语从句中做宾语可以省略