be动词一半动词,介系词后都是受格吗?

人称代词的受格在句中的功用，第一是作及物动词的宾语，比如： I saw you in the street. 也可以作介系词的宾语，如： This pen is bad. I can't write with it. 在动词be后作表语的人称代词，一般用受格，如： It's me. Was that her? 但是跟有who或that引导的从句（子句）时用主格，如： It is I who did it. 又如： I didn't expect the winner to be me. I would like to be him.