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

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为什么不能只用that或只用which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