A)人称代词有主格和宾语之分：主格用作主语和表语，宾格用作宾语。
如: I like music(主语). She is a teacher. (主语)
She and I are good friends(主语). Neither she nor I am student.
--I saw the boys this morning.
--Are you sure it was they(表语)?
B) 人称代词的宾格在句子作动词的宾语，或者介词的宾语。
如：I saw him at the party(动词see的宾语).
I havent seen them recently. (动词see的宾语)
I bought a book for them.(作介词for的宾语)
=I bought them a book.(作介词buy的间接宾语)
☆注意：
a.在口语中，人称代词的主格形式作表语时，常常可以用宾格替换。
如：It cant be he/him.
--Is this Mr. Green?
--Yes, this is he/him.
b. 在口语中，当人称代词用于than, as 之后或用于强调句中被强调时，用主格 (较正式) 和宾格 (较口语化) 均可以。
如：
He sings better than I/me. 他比我唱得好。
You know more than she/her.
She is as tall as me (me=I am).
c. 单独使用的人称代词通常用宾格，即使它充当的是主语也是如此。如：
I like English. Me too=I like English too. 我喜欢英语。我也喜欢。
--Who did it?
--Them.(=They did it)