I remembered a rule from a forgotten English teacher from the 50’s and I want to check: after I use a “wish” or “if” is it necessary to change a subsequent “was” to a “were”?

And here’s our answer:

As with most things in life, it depends.

The key point in deciding whether to use “was” or “were” is whether or not you’re talking about something that’s true (or probably true).  If it’s true (or probably so), use “was”; if untrue (or probably so), use “were.”

So if you say “He thinks that if he was/were somewhere else, he’d be rich and happy,” you’d use “were.”
But if you say “If that noise was/were the phone, I didn’t realize it,” you’d use “was.” In the first example, it’s not true that he’s somewhere else; in the second, it appears likely that the noise was the phone.

What we’re dealing with here is what grammarians call “moods” or “mode,” the use of different verb forms to show the speaker-writer’s intent. The indicative mood is used for ordinary sentences like simple questions or statements of fact. The main use of the subjunctive mood is to show “conditions contrary to fact.”

So someone who says “If the mailman were here...” is indicating that the mailman is not present—”If the mailman were here, I’d give him this letter.” But saying “If the mailman was here...” suggests that the mailman may well have been there—“The mailman has almost always comes by before this time, but if he was here, he left no mail.” 

We used to see the subjunctive much more frequently, but for present-day writers and speakers, the most important thing to know is what’s been covered here—use “was” with factual things, “were” with non-factual.