I often see these following errors:
- If he was
- If I was
- If she was
The correct forms should be:
- If he were
- If I were
- If she were
Why is this so? This is because these phrases are in the subjunctive mood. The subjunctive mood is used to express a state of affairs that is contrary to the facts. It can also states a wish or desire that is not true.
For example, I could say, "I wish I were a millionaire" (I'm not); "If she were pretty" (she's not); "It this were true" (it's not); "If it were sunny we could go out" (but it's actually raining); "She spoke as if I were deaf" (but I can hear perfectly well).
In the above examples, we're imagining a state of affairs that isn't true. This is the subjunctive mood. Grammar rules dictate that the plural form "were" should be used with a singular subject. Therefore the correct form is "If I were" and not "If I was".
Bonus: remember that the personal pronoun you is always paired with were. Thus it is always "You were" and never "You was".
Extra credit section: "If I was him" contains two errors. The first of course is that since this is the subjunctive mood, it should be were and not was. Secondly, it should be he and not him. In other words, it should be "If I were he". We use he in this case because were is a form of the verb to be and when using to be the nominative case pronoun I must be matched in type and so we have to use he. (I, she, he, we, they, who are nominative case pronouns while me, her, him, them, whom are objective case pronouns.)