By Marina Pantcheva
Whether and if are often interchangeable, but sometimes only one of them is correct. To avoid any danger of mistake, use if to introduce a condition and whether when there are two alternatives possible.
If is used to introduce a condition.
(1) If it does not rain tomorrow, we will go to the beach.
(2) Please contact us if you have further questions.
Whether and if are interchangeable when used to report a yes/no question.
(3) I am not sure whether/if the train has already arrived. (Has the train arrived? – yes/no)
(4) We wonder whether/if it will rain tomorrow. (Will it rain tomorrow? – yes/no)
In formal writing we always use whether in cases like (3) and (4).
In some cases, however, whether and if are not equivalent.
Use whether to present two alternatives.
(5) Inform the student adviser whether you need help.
In example (5), there are two possibilities: you need help or you do not. In either case you must inform the student adviser. This contrasts with the use of if.
(6) Inform the student adviser if you need help.
The example in (6) means that you must inform the student adviser only in case you need help. The need for help is a condition for informing the student adviser.
Another contrasting pair:
(7) Let me know whether the printer still jams. ( = no matter whether the printer jams or not, you should let me know)
(8) Let me know if the printer still jams. ( = in case the printer jams, let me know about it; if the printer does not jam, do not let me know about it)
Use whether before an infinitive.
(9) We’ve been wondering whether to apply for this grant.
Use whether when the clause beginning with whether is the subject of the sentence.
(10) Whether we win is irrelevant.
Whether or not means “regardless of whether”
(11) I will complain whether or not the committee accepts my application. ( = no matter what the committee does, I will complain.)
In all other cases “or not” is superfluous.
(12) We’ve been wondering whether or not to apply for this grant. [bad language]
(13) Let me know whether or not the printer still jams. [bad language]