When describing something that is contrary to what is true, you cannot use the regular past tense of the verb to be. You must, instead, use the subjunctive. The subjunctive is necessary in many situations, but I’ll discuss two of them here: with the verb to wish (which always expresses what is contrary to fact) and with the word if when it is expressing something that is contrary to fact.
When someone says If I was a columnist in Indianapolis, I would write that story, he is saying that he is not a columnist in Indianapolis, so that’s an example of using if to express something that is contrary to fact. Therefore, using the past tense—was—is incorrect. He should use the subjunctive: If I were a columnist in Indianapolis, I would write that story.
When using the subjunctive, the form of the verb to be is were, regardless of the subject. Here are some other examples.
I wish I were a columnist in Indianapolis.
If she were a columnist in Indianapolis, then she wouldn’t live in Tampa.
He wishes that he were a columnist in Indianapolis.
If it weren’t 1000 miles from Tampa to Indianapolis, then there would be no story.
If he was too critical of the article, then he’s probably sorry. (Here, was is correct because this statement is not necessarily contrary to fact; he may very well have been too critical, so we use the regular past tense, which is was.)
If I was driving a blue car, then it must’ve been more than five years ago. (It may very well have been a blue car, so was is correct.)
Again, just remember that you use the subjunctive when you’re talking about something that is definitely contrary to fact.