1. If I were you, I'd buy a new baseball cap.
The "were" in #1 is an example of the past subjunctive, which uses "were" in all persons, singular and plural.
The past subjunctive in an if-clause expresses a "contrary to fact" situation ("I" am not "you"). It doesn't have temporal significance.
You could explain it as a past tense form, rather than a past tense; just as the past tense form in #2 suggests speculation, rather than real past-ness:
2. If you bought a new baseball cap, women would find you irresistible.
Does that help at all?
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