them是他们,第三人称代词复数宾格形式作宾语如:I like them.

whom是谁,人称代词疑问词单复数同形,作宾语如:Whom should I told

who是谁的意思,人称代词疑问词单复数同形,作主语如:Who is your sister

which是哪一个的意思,单复数同形指示代词如:Which is your bike