As a supplement:
Originally Posted by anasiants
1. The sentence needs rewording slightly, even with "was":
"If I was Supergirl, I would have slapped your hands a long time ago for behaving (so) badly."
2. Not every case of "was" in an IF-clause can be changed to "were" – only those that relate to an "unreal" hypothesis. For instance:
You are in a supermarket with a friend. You see a man buying cigarettes and leaving the supermarket. You say to your friend:In this case, the conditional sentence doesn't relate to an unreal hypothesis: you are assuming he was the man. So you can't change "was" to "were".
"That looks like the man the police want to interview, in connection with the missing subjunctive."
Your friend says:
"Yes, I remember now. I saw his picture on the 10 o'clock news. What shall we do?"
And you say:
"Well, if that was the man on the 10 o'clock news, we'd better inform the police."