"If I were you" is correct according to grammar but,"If I was you" is not incorrect because I have heard some people use it generally but,we'd better use the thing that is according to rule.

If I were you is correct

If you really want to know WHY "If I were ..." is correct, you will have to examine the Subjunctive verb form. Try this website: http://www.englishclub.com/grammar/verbs-subjunctive.htm Here is a short excerpt from that site: ====================================================== Why do we say "I were", "he were"? We sometimes hear things like "if I were you, I would go" or "if he were here, he would tell you". Normally, the past tense of the verb "to be" is: I was, he was. But [in FORMAL speech] the "if I were you" structure does not use the past simple tense of the verb "to be". It uses the past subjunctive ... FORMAL: If I were younger, I would ... FORMAL: If he weren't so mean, he would ... FORMAL: I wish I weren't so slow! INFORMAL: If I was younger, I would ... INFORMAL: If he wasn't so mean, he would ... INFORMAL: I wish I wasn't so slow! ====================================================== As noted, the "I were" structure is correct for formal / professional usage. However, in day-to-day average American conversation, you will hear "I was" more frequently. Of course, the real reason WHY is ... 英文很神经病。

If I were you

I am not an expert on English grammar but I think you use 'were' instead of 'was' here to express uncertainty. I am not you, and I only give an opinion. You could also use 'would be', by the way. You could compare it with Konjunktiv in German or subjuntivo in Spanish. In those two languages the use of special verb form to express uncertainty is far more common than in English. (And far more common than in Dutch my mother language.) Like Ali said though, we can also understand you if you would use 'was'.