题目内容：

Thw knife___we cut the bread is very sharp.为什么不能用with that?

优质解答

Thw knife_with which__we cut the bread is very sharp
当介词置于关系代词前时,指物只能用which ,指人只能用whom

追问：

为什么不能只用that或只用which

追答：

约定俗成的，记得就是。 Thw knife_with which__we cut the bread is very sharp =Thw knife which(或者that)_we cut the bread with is very sharp =Thw knife_we cut the bread with is very sharp