Q. Which is correct, “If I were you . . .” or “If I was you . . .”?
A. Write “if I were you.” This is an example of the present subjunctive, which uses the past-tense “were” to express something that’s impossible or hypothetical. The past subjunctive uses “had”: e.g., “if I had been you.” The construction “if I was you” should be considered colloquial. See paragraphs 5.117, 5.137, and 5.140 in CMOS (i.e., chapter 5, on grammar, by Bryan Garner) for more on the subjunctive mood.