Grammar: when to use ‘whether’
Lower intermediate to lower advanced level (B1/C1)

In many cases you can use ‘whether’ or ‘if’ and your sentence will be grammatically correct but in some cases you can only use ‘whether’.

After prepositions
In the sentence: ‘He’s interested in green energy.’ ‘in’ is a preposition. Because of this, in the following sentence you can use ‘whether‘ but not ‘if‘:

He’s interested in whether green energy can lower the cost of electricity.

Here are some examples with different prepositions:

Tell me about whether you’ve decided to take IELTS (or not).
It boils down to whether you are prepared to take the risk (or not). (= the most important thing to consider is …..)

In the above it is difficult to think of a way to express the same meaning with ‘if’.

Before a ‘to-infinitive’ (to go, to eat, to help, etc)
Some examples should help:
I’m debating whether to take the last train or travel tomorrow (not ‘if’).
Whether to tell him or not is what I have to decide.
She doesn’t know whether to buy the presents now or wait until the sales.

You can usually use ‘if’ too but you will need to change the sentences a little, for example:
I’m debating if I should take the last train or travel tomorrow.
She doesn’t know if she should buy the presents now or wait until the sales.

You’ll notice that to avoid the ‘to-infinitive’ you can often use ‘should’.

With ‘or not’
Often we want to emphasise alternatives by using the phrase ‘or not‘, for example:
I don’t know whether or not I should go. We can also say: I don’t know whether I should go or not. However, if you choose to use ‘if’ the ‘or not’ needs to come at the end, for example, I don’t know if I should go or not.

In cases where ‘whether’ and ‘if’ are both possible, ‘whether’ is usually the more formal choice.