If vs Whether

If and whether: indirect questions

We can use if or whether to report indirect yes-no questions and questions with or. If is more common than whether:

Ex
Call the bakeries around town and find out if any of them sell raspberry pies.

I rang Peter from the station and asked if I could drop in to see him before going back or if he’d meet me.

I don’t know if I can drive. My foot really hurts.

I’m not sure whether we’ll be able to learn very much from them.

We often prefer whether in more formal contexts:
The teachers will be asked whether they would recommend the book to their classes.

Whether not if
We use whether and not if after prepositions:

Ex

Later I argued with the doctor about whether I had hit my head,
The police seemed mainly interested in whether there were any locks on the windows.

We use whether, not if, before to-infinitives:
Ex
I don’t know whether to buy the blue one or the red one.
Not: I don’t know if to buy the blue one …

We use whether, not if, directly before or not:
Ex
Can you tell me whether or not you’re interested in the job.
Not: Can you tell me if or not you’re interested …

We use whether, not if, after prepositions:
Ex
[talking about a trip to Australia for a year]
We’re not interested in whether we get great jobs and that kind of thing, we just want to have a good time.
Not: We’re not interested in if we get great jobs and that kind of thing …

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If Versus Whether

Here's an example where the two words could be interchangeable:

Squiggly didn't know whether Aardvark would arrive on Friday.
Squiggly didn't know if Aardvark would arrive on Friday.

In either sentence, the meaning is that Aardvark may or may not arrive on Friday.

Now, here are some examples where the words are not interchangeable:

Squiggly didn't know whether Aardvark would arrive on Friday or Saturday.

Because I used whether, you know that there are two possibilities: Aardvark will arrive on Friday or Aardvark will arrive on Saturday.

Now see how the sentence has a different meaning when I use if instead of whether:

Squiggly didn't know if Aardvark would arrive on Friday or Saturday.

Now in addition to arriving on Friday or Saturday, it's possible that Aardvark may not arrive at all. These last two sentences show why it is better to use whether when you have two possibilities, and that is why I recommend using whether instead of if when you have two possibilities, even when the meaning wouldn't change if you use if. It's safer and more consistent.

Here's a final pair of examples:

Call Squiggly if you are going to arrive on Friday.
Call Squiggly whether or not you are going to arrive on Friday.

The first sentence is conditional. Call Squiggly if you are going to arrive on Friday means Aardvark only needs to call if he is coming.

The second sentence is not conditional. Call Squiggly whether or not you are going to arrive on Friday means Aardvark needs to call either way.

To sum up, use whether when you have two discrete choices or mean "regardless of whether," and use if for conditional sentences.