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

将从句补充完整应该是we cut the bread with the knife.
that和which都是替代一个名词的,这里显然要替代 with the knife,不符的,所以不能用为什么不能替代？that 和which是替代名词的，例如句中的knife，可是我们能看到这句子缺的是with the knife如果要用that 和which,应该是The knife with which we cut the bread is very sharp或者是The knifethat we cut the breadwith is very sharp