The Quick Answer

Use if to introduce a condition. In all other circumstances, use whether. (This ruling will see you right, but there are other options.)

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Click on the Two Correct Sentences
(Interactive Game)

If and Whether

Whether and If (Interchangeable)

• I am unsure whether I will be attending the party.
• I am unsure if I will be attending the party.
(In this example, the yes/no question is "Am I attending the party?")
• Janice wondered whether she had unplugged the iron.
• Janice wondered if she had unplugged the iron.
(In this example, the yes/no question is "Did Janice unplug the iron?")

• I would like to know whether it is a true story or fabricated.
• I would like to know if it is a true story or fabricated.
Note: Using whether is far more common.  It is certainly more formal.

Whether

• Inform the clerk whether Mark needs a seat.
(In this example, the two alternatives are Mark needs a seat and Mark does not need a seat. The clerk is to be informed in either case.)
• Inform the clerk if Mark needs a seat.
(This sentence is not grammatically wrong, but it does not mean the same as the first example. In this example, the clerk is only to be informed if Mark needs a seat. Therefore, Mark needing a seat is a condition. This is a conditional sentence. See the section on If below.)
• Let Anna know whether the boss is able to go to Crowborough.
(In this example, the two alternatives are going and not going.  Anna needs to know the answer regardless of which is chosen.)
• Let Anna know if the boss is able to go to Crowborough.
(This sentence is not grammatically wrong, but it does not mean the same as the one above. In this example, Anna needs only to be told if the boss is going to Crowborough.)

• I would like to talk about whether you are going to California.
(The word about is a preposition.)
• At this point, the flight attendant makes the decision on whether the passenger stays on the aircraft.
(The word on is a preposition.)

• I have been thinking whether to grow my own tomatoes this year.
(To grow is an infinitive verb)

• Whether you sink or swim is not my concern.
(Whether you sink or swim is the subject of this sentence.)
• I don't care whether you sink or swim. 
(Whether you sink or swim is the complement of the verb to care.)

• I doubt whether the team will succeed.
• Please establish a committee to determine whether the proposed funding lines are appropriate.

If

• If you sing, I'll pay you ten pounds.
• Peter will catch you if you fall.

Whether or Not

• Reprimand Chris whether or not he is on time today.
(...regardless of whether he is on time today.)
• The parade will go ahead whether it rains or not.
(...regardless of whether it rains.)
(Note: There is leniency on where the or not is placed. It does not have to follow immediately after whether.)