1英语语法题(1)mr.liu is the person(whom) you talked about on the

1英语语法题
(1)mr.liu is the person(whom) you talked about on the bus.
为什么这里的whom可以省略.
(2)mr.liu is just the boy whom i want to see .
这里的WHOM就不省呢?
(1)football is a fame which is liked by most boys.
这里的WHICH就没省略
(2)this is the pen (which )he bought yesterday
这句为什么就可以省略了呢