There are subtle differences in the way we use 'whether' and 'if'. At times, it might seem as if we can use them interchangeably, but there are specific situations when one is more appropriate than the other.
1. When both choices are given, we generally use whether rather than if:
When we find ourselves considering two options or alternatives, the word 'whether' tends to be the preferred choice. While 'if' can also be used in such situations, 'whether' is more commonly employed. For example:
I don't know whether/if she's coming or not.
In this instance, using 'if' would still convey the intended meaning, but it's less common tnan using 'whether'.
2. Before an infinitive with to we use whether:
When we encounter an infinitive with 'to,' such as in the expression of our intentions or decisions, 'whether' becomes the appropriate selection. For example:
I don't know whether to accept their offer or not.
If would not be used here. Instead, 'whether' conveys the sense of deliberation or hesitation surrounding the decision.
3. We can use whether after a preposition:
In certain instances, we employ 'whether' after a preposition to articulate a question or concern. For example:
It's a question of whether we can agree on everything.
If would not be used here. Instead, we use 'whether' to highlight the inquiry or uncertainty about reaching a consensus.
4. Both whether and if can be used in indirect speech to introduce a yes/no question:
Both 'whether' and 'if' can be used in indirect speech to introduce a yes/no question. For example:
Did they say whether/if they were going to be late?
Here, both 'whether' and 'if' can be used.
5. After some verbs, we use whether:
Certain verbs are commonly followed by 'whether' to express doubt or contemplation. For example:
I doubt whether they'll make it.
We discussed whether it was the correct decision.
If would not be used here.
Check the usage on Google below, or look up the definitions on Dictionary.com:
Google.com results: Of whether | Of if | Whether to | If to
Dictionary.com definitions: Whether | If