If I was older, I'd leave school. 和 If I were you, I'd go to the dentist. 的差別在哪裡？如果不好說明，請提供一些例句。

Both are correct.

The difference in use of "was" and "were" is due to the history of English. We used to have something called a "subjunctive mood," which still survives in some constructions. Here's an example: "I wish I were prettier." According to the commonly-followed rules today, we should say "I wish I was prettier." The subjunctive was traditionally used for expressing wishes or desires. Both are still heard today, and there is no difference in meaning between them.

Likewise, we meet the remnants of the subjunctive mood (principally in the verb "to be") in the second conditional. It's good to know that there is no difference in meaning. You could just as easily say "If I were older, I'd..." or "If I was you, I'd..."

Hope that helped.