Question

When do we use "If I were you" and "If I was you"? Thanks in advance :)

Answer 1

“If I were you” is subjunctive,Subjunctive is difficult even for most native English speakers, mainly because it’s not a tense; it’s a mood. Past, present, future—those are tenses.The word were in the phrase if I were you is special form. It is known as the subjunctive mood (from the grammatical point of view).

Today you also find the phrase if I was you. Here the Simple Past form of be is used. But there are people who say that this phrase is incorrect and would never use it (mainly Americans). Others say that this phrase can be used

Answer 2

If I were you, I would + (verb)

If I were you, I would enjoy my vacation.

If I were you, I would explain what happened.

If I were you, I would continue working until it is done.

If I were you, I would book my reservations now.

If I were you, I would answer the question.

....

If I were you, I would + have + (verb the 3rd.form)
By adding 'have' after the word 'would' you are talking about something in the past tense.

If I were you, I would have enjoyed my vacation.

If I were you, I would have explained what happened.

If I were you, I would have continued working until it was done.

If I were you, I would have booked my reservations now.

If I were you, I would have answered the question.

...

source: http://pl.talkenglish.com/LessonDetails.aspx?ALID=2067