"Was" or "were" in subjunctive clauses

I'm not a native english speaker, so even though I'm decently proficient at it, I don't really "know the rules" sometimes, and this is one that's been confusing me for a long time.

Which one is correct in each sentence?

If the movement [was/were] to continue uncorrected, the tower would
one day topple.

If I [was/were] rich, I would buy a yacht.

NOTE: I care not only about the case of "I", but also "she", "them", "it", etc, as in the example of the tower. Would it be any different if instead of the tower, it'd be me who'd topple if uncorrected?

I'm pretty sure it's "were" in both cases. That's what they taught me, I think.
I started to doubt when I saw a lot of "was", but it sounded like the typical intentional mistake used "stylistically". ("If I was a rich girl...")

Then I saw it some more and thought it came down to an American/British English difference (I was taught British, in theory, and most of what I read is American).

But that tower sentence came straight from "The Guardian"...

When do you use was and when do you use were?

Answer

The grammatical rule, if you want to be strict, is that in subjunctive clauses you always use were, therefore all of the following examples are correct:

If I were you, I'd definitely think this through.
If she were to know what you did, she'd be so angry!

However, some people break this rule, to me for reasons unknown. Either they are unaware of it (insufficient grammar on their side), or they are being informal, or non-native speakers who were never taught the rule.

Just use were in all cases and you're fine.

Also note that as FumbleFingers has correctly mentioned:

It's generally accepted that use of the subjunctive is declining, so
eventually it will disappear. Some publications will be ahead of the
curve, and there's no reason why The Guardian shouldn't be one of
them. As Colin says, many of us still make the formal/informal
distinction, but increasingly this is seen as just a matter of style,
rather than correctness.