When we deal with two pronouns at the same time in a statement, we use both pronouns either in subjective or objective case: ‘He and I arranged the party’; ‘if you don’t want to go alone, you can take either her or me along’, ‘she and I are schoolmate.’
He and I are old buddies. (NOT He and me are old buddies)
The boss and I were in the office.
The teacher made a copy of the project for Kelly and me.
Jack and I are planning a visit to grandma.
We use subjunctive when we want to express wishes or desires. Example: I wish she were my teacher.
In the case of ‘If I were he/ him…’, we need the subject pronoun because it is the subject complement and not the object. The rule says the pronoun that follows a linking verb is and should be a subject complement, not an object. Here, ‘were’ is a linking verb and needs predicate nominative after it. Like, ‘This is he’. So, we’ll take the subject pronoun ‘he’ and not the object pronoun ‘him’: ‘If I were him, I would do the same’; ‘if I were she, I would dump that loser’.
However, it would be curious to know in the case of ‘It’s me’ or ‘It is I’, where you could also correctly use the subject pronoun ‘It is I.’ Though the sentence with subject pronoun seems absolutely correct, it doesn’t sound good to ears.
Some examples of ‘if I were he’
If I were he, I would resign from that position.
He walked to the station; if I were he, I’d take a taxi.
I wouldn’t worry too much about that stupid girl if I were he.
I’d definitely apply for that job if I were he.
I wouldn’t give it too much importance if I were he.
So if I were he, I’d just leave the matter aside and talk about the future.
I would take that ‘racist’ remark too seriously if I were he.
I think it counts as ‘average’, and I wouldn’t worry if I were he.
I’d give her a call if I were he, and work something out.
If I were she, I would wear pink eye shadow.