"before you leave" vs. "before you left"

In the movie, Good Will Hunting, the main character, Will, says to his ex-girlfriend on the phone, "I just wanted to call you up before you left." and the past tense "left" sounded strange to me as his ex-girlfriend has NOT left yet when he says that.

Which of the followings is grammatically correct? 1. I just wanted to call you up before you left. 2. I just wanted to call you up before you leave.

English has a sequence of tenses rule, whereby the tense in an adverbial clause (in this case "before you left") is partly governed by the tense in the matrix clause (which in this case is the whole sentence, whose central verb is "wanted").

Specifically, when the matrix clause is in the past tense (e.g. did) and/or the perfect aspect (e.g. have done) and/or the conditional mood or irrealis mood (e.g. would do or did, respectively), then the adverbial clause will almost always be in the past tense (e.g. did); otherwise, it will almost always be in the present tense (e.g. do). Interestingly, this means that adverbial clauses are almost never in the future tense; for example, we say e.g. "I'll call you up after you leave" (or "I'll call you up after you've left", with the present perfect), not *"I'll call you up after you'll leave".

This is not always a 100% hard-and-fast rule, but it's really pretty close, and I don't think you'll ever go wrong following it. In the specific example you quote, though, I think "before you leave" would also have been OK.

The same rule is often followed with relative clauses ("I knew a woman who lived there", even if the woman still lives there), and to a lesser extent interrogative clauses ("I often wondered who lived there", ditto), but with both of those it's more flexible. That has the interesting effect that native speakers will sometimes follow a sequence-of-tenses rule for a relative clause and therefore use the past tense, but then awkwardly "correct" themselves with the present tense; so you'll hear things like, "I used to date a guy who worked there. I mean, he still works there, but we're not dating anymore."