You see this all the time with who/whom, me/I, and other similar word pairs. Another area of confusion is the difference between was and were. Most people think they understand the difference, but there’s so much more to it.
Was/were is the “past tense” version of “to be.” I am in my office, but I was at the coffee shop an hour ago. That seems simple enough. It’s also generally understood that “was” is singular and “were” is plural. She was in Tokyo, but they were in Montreal.
But there’s more to it than that.
One construction that has fallen to the wayside is something called the “subjunctive” form. This is to refer to the conditional future or to something hypothetical, unlikely, or currently untrue. Let’s take a look at a few examples.
If I was a professional basketball player, I would be rich.
This is incorrect, because you’re not really talking about the past. You are talking about the present (or a conditional future). Instead, the correct construction would be:
If I were a professional basketball player, I would be rich.
This is a hypothetical situation, either in the present or in the future. Similarly, you could consider a sentence like this:
I wish I were a little bit taller.
Again, that is a current hypothetical situation. By contrast, take a look at this sentence that addresses the past:
I wish I was taller as a kid. Then, I could have played on the basketball team.
Do you see the difference? Realistically, you could use “was” in casual speech under both circumstances and most people wouldn’t call you out on it. It is not as significant a difference as, say, ever/every so often, but for more formal speech and writing, it’s always best to be as correct and proper as possible.