The thief had to give everything [ ] he had stolen to the police .【 】填什么

that 和which在定语从句中都是做关系代词,指代先行词是事物的名词或代词（that还可以指代人,which 不可以）.通常情况下二者可以互换.
区别：
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's all that he told me.
2.先行词前有the only,the same,the very 或no,little,much,any,every,all 等修饰时,关系代词应用that
She is the only person that I know in this company.
3.先行词前有序数词或形容词*修饰时,关系代词常用that
The first thing that I want to do is to take it away.
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the same...that...; so...that...; such...that...以及it is/was...that...的强调句型中,要用that
This is the same park that we once visited.
6.如主句以there be开头时,关系代词应用that
There is a room that has two windows.你还没回答我【stolen】什么意思呢，(⊙v⊙)嗯stolen 是steal 的过去完成时， |“偷窃”的意思、这个没有看到哈