In informal writing and speech if and whether are often interchangeable.
– ‘I don’t know if he’s coming’ = ‘I don’t know whether he’s coming’.
In formal or technical writing the meaning can sometimes be different depending on which word you use.
Use if when you have a conditional sentence and whether when you are showing that two alternatives are possible.
IT’S NOT WHETHER YOU GET KNOCKED DOWN, IT’S WHETHER YOU GET UP. Vincent Lombardi
In the following example the two words could be interchangeable:
– Peter didn’t know whether Brendan would arrive on Saturday.
– Peter didn’t know if Brendan would arrive on Saturday.
(The meaning is the same: Brendan may or may not arrive on Saturday.)
Now look at these examples:
– Peter didn’t know whether Brendan would arrive on Saturday or Sunday.
(There are two possibilities: Brendan will arrive on Saturday or Brendan will arrive on Sunday.)
– Peter didn’t know if Brendan would arrive on Saturday or Sunday.
(He may arrive on Saturday or Sunday, but he may not arrive at all.)
It is therefore better to use whether when you have two possibilities.
ONLY USE ‘WHETHER’
Before infinitives with ‘to’
– ‘I don’t know whether to tell him’.
– ‘Is the trip worth it? It depends on whether you like that kind of music.’
‘Whether’ is also preferred when the clause beginning with ‘whether’ is a subject or a complement.
– ‘Whether you agree or not makes no difference at all’.
With ‘or’. This used to be unusual, but is quite common now.
– ‘I didn’t know whether I should laugh or cry’.
USING ‘OR NOT’ AFTER WHETHER
– ‘Call Peter if you are going to arrive on Saturday’.
This sentence is conditional. It means you only need to call if you are coming.
Leave out ‘or not’ when it is something superflous such as in this example:
– ‘Peter didn’t know whether Brendan would arrive on Saturday (or not)’.
Use ‘or not’ when it means ‘regardless of whether’:
– ‘Call Peter whether or not you are going to arrive on Saturday’.
This means you need to call either way.