'Whether' or 'If'

'If' is used to introduce a conditional idea or an idea that has just one possibility whereas 'Whether' is used to introduce alternative possibilities, usually with 'or not' implied or explicitly stated in the sentence.

Consider the following example:

If you study hard you will surely do well on the GMAT.

In the above sentence 'If' is introducing a single condition (if you study hard) that can lead to the desired goal; we can't replace it with 'Whether' because that would totally distort the meaning of the sentence.

Now look at this one:

I can't decide whether to study or go out with my friends.

Here 'Whether' is introducing two alternatives and we cannot be replace it with 'If'.

To make it clearer here is a sentence in which I've used both 'If' and 'Whether' together:

You need to decide whether you will be able to control your temper if he asks you to get out of his office.

To Sum it Up:

1. 'If' is used to introduce 1 condition whereas 'Whether' is used to introduce several possibilities.

   
   
2. If you are still confused on a question, go with 'Whether' since it's preferred on the GMAT.