gmatrant wrote:Which of the below two is correct and when do you use one for the other.

a) If I were you then I would mail the postcard
b) If I were you then I will mail the postcard

This is confusing to me. Kindly comment with a clear explanation.

B is awkward!
--- break it into a smaller sentence.

If x....I would
If x....I will

"I would..." or "I will" are dependent clauses on the "If" clause. Since the latter is dependent, you use would. Will suggests that you will definitely do something. But that's not true. You WOULD do it, IF X happens.

A is better!

Gurpinder is correct. A is the proper form.

This is a time-shift (or hypothetical) version of the subjunctive. The "were" in "If I were you" is the subjunctive form of the verb to "be". The indicative form of the singular first person of this verb is "am." This would be "I am you." But this is not true, so the statement uses the hypothetical, "If I were you."

The time shift subjunctive requires the conditional form of the verb. As Gurpinder said, "will" is too definite given that we are speaking of the hypothetical. "Would" is the correct conditional form of the verb. In the situation where I am you - -then I would mail the postcard.

Thanks for clarifying Gurpinder & David.

Hi!

If I were you then I would mail the postcard.

This sentence is an example of type 2 Conditional. It is used to express imaginary situations which are contrary to facts in the present and, therefore, are unlikely to happen in the present or future.
the rules:
If-clause
If + past simple/ past continous
Main clause
would/could/might + present bare infinitive

If I were you then I will mail the postcard.

This sentence is confusing. Either it is incorrect or it is mixed conditional( type 2 if-clause & type 1 main clause).
I don't know whether GMAT permits mixed condtionals.

Source: Grammar 4, Enterprise Student's book

Hope it helps!
Best,
Maciek