http://www.grammar-monster.com/easily_confused/if_and_whether.htm

If / Whether--Is it a condition or an alternative?

A Condition vs. an Alternative

A CONDITION

AN ALTERNATIVE

In many cases, if and whether can be used interchangeably without affecting the meaning.  Below are some subtle differences in formal use. Use if to indicate one condition that requires an action. If X is true, then do Y.

Use whether to indicate two conditions, alternates, that require an action: yes or no,  X or Y.  If X or Y is true, then do Z.  (Note that both if or whether are commonly used in an embedded question.)

Let me know if you get cell phone reception.  (one condition)
 

Let me know if / whether you can get cell phone reception.  (yes or no – either condition)

He asked me if I had an idea.  (one condition)

 

He asked me if / whether I had an idea or a plan. (either condition)

    Two Alternatives Options

IF

WHETHER

If – is less commonly used when both sides of an alternative are given.

Whether – is more commonly used when alternatives are stated.

He asked me if my phone was receiving cell service or not. (informal)

He asked me whether my phone was receiving cell service or not.

He asked me if I was using ATT or Verizon mobile service. (informal)

He asked me whether I was using ATT or Verizon mobile service.

 After a Preposition

VERB + PREP + IF — NOT USED

VERB + PREP +WHETHER

If – is NOT USED after a verb + preposition phrase introducing an alternative.

Whether – is used after a verb + preposition phrase introducing an alternative.

*I was concerned about if we could get cell phone reception. (NOT used)

I was concerned about whether we could get cell phone reception. 

*We were interested in if we could receive cell service on the mountain top.  (NOT used)

We were interested in whether we could receive cell service on the mountain top. 

*We're not sure about if a cell phone would work there.  (NOT used)

We're not sure about whether a cell phone would work there. 

* Words are examples of incorrect usage.

 
Before an Infinitive

IF

WHETHER

If – is NOT USED  before an infinitive phrase introducing an alternative.

Whether – is sometimes used before an infinitive phrase introducing alternatives.

*I can't decide if to move to the right or to the left.  (NOT used)

I can't decide whether to move to the right or to the left.

*It was unclear if to stand in the middle of the room or near the window.(NOT used)

It was unclear whether to stand in the middle of the room or near the window.

 In Indirect / Reported Questions

IF

WHETHER

If – is commonly used in indirect questions.  Some formal usage restricts if to introducing conditional clauses.

Whether – is commonly used in indirect questions to introduce an alternative (whether or not)

I DON'T KNOW IF…

I don't know if my cell phone will work here. 

I DON'T KNOW WHETHER…

I don't know whether my phone will work here.
STATEMENT: I don't know QUESTION: Will my phone work here?

I don't know whether my cell phone will work if I use it here.
       (alternative - whether or not)                           (condition)

 

DO YOU KNOW IF…

Do you know if there is cell phone service in Yosemite Park?  

 

 

DO YOU KNOW WHETHER…

Do you know whether there is cell phone service in Yosemite Park?
STATEMENT: Do you know QUESTION: Is there cell phone service in Yosemite?

Do you know whether there is cell phone service if standing in Yosemite?
                     (alternative - whether or not)                      (condition)

 

 

 

Formal  Contexts

IF

WHETHER

If – is not commonly used in formal contexts when introducing an alternative clause. (This is an unusual, old rule that is still included in college English entrance exams.)

Whether (or not) introducing an alternative clause is used both formally and informally.

*The President declined to say  if  he would give up his personal Blackberry. (formal context)
My friend didn't say if she would give up her cell phone.   (informal context)
  

The President declined to say whether he would give up his personal Blackberry.

*The Congressman was uncertain  if   the health bill would pass. (formal context)
Bob was uncertain if his if his son would pass the 5th grade.   (informal context)

The Congressman was uncertain whether the health bill would pass.

*We discussed  if  we would lay off employees.

We discussed whether we would lay off employees.

*The members hadn't settled on   if  they would accept the offer.

The members hadn't settled on whether they would accept the offer.

 Initial Sentence Position

IF

WHETHER

If – is not used in a clause at the beginning of a sentence when introducing an alternative. (This is not true for conditional clauses.)

Whether – is more commonly used in a clause at the beginning of a sentence when introducing an alternative (whether or not).

If the battery is charged is my biggest concern.  (NOT used)
My biggest concern is if the battery is charged.  (alternative - less commonly used)
If the battery is charged, my phone works well.  (condition - commonly used)

Whether the battery is charged is my biggest concern. (alternative)
My biggest concern is whether the battery is charged.  (alternative)
Whether or not the battery is charged my phone doesn't work. (no condition exists - even if)
 

If my cell phone works there is a mystery to me.  (NOT used)
It is a mystery to me if my cell phone works there.  (alternative - less commonly used)
If my cell phone works there, I'll be surprised.  (condition - commonly used)

Whether my cell phone works there is a mystery to me. (alternative)
It is a mystery to me whether my cell phone works there.  (alternative)
Whether or not my cell phone works there, I won't accept calls. (no condition exists - even if)
 

 

http://www.grammar-quizzes.com/if-whether.html

Part 2--Additional Explanation

 First, let's figure out when to use whether and when to use if.

'If'   Versus    'Whether'

Although in informal writing and speech the two words are often used interchangeably, in formal writing, such as in technical writing at work, it's a good idea to make a distinction between them because the meaning can sometimes be different depending on which word you use. The formal rule is to use if when you have a conditional sentence and whether when you are showing that two alternatives are possible. Some examples will make this more clear.

Here's an example where the two words could be interchangeable:

Squiggly didn't know whether Aardvark would arrive on Friday.

Squiggly didn't know if Aardvark would arrive on Friday.

In either sentence, the meaning is that Aardvark may or may not arrive on Friday.

Now, here are some examples where the words are not interchangeable.

Squiggly didn't know whether Aardvark would arrive on Friday or Saturday.

Because I used whether, you know that there are two possibilities: Aardvark will arrive on Friday or Aardvark will arrive on Saturday.

Now see how the sentence has a different meaning when I use if instead of whether:

Squiggly didn't know if Aardvark would arrive on Friday or Saturday.

Now in addition to arriving on Friday or Saturday, there is the possibility that Aardvark may not arrive at all. These last two sentences show why it is best to use whether when you have two possibilities, and that is why I recommend using whether instead of if when you have two possibilities, even when the meaning wouldn't change if you use if. It's safer and more consistent.

Here's a final pair of examples:

Call Squiggly if you are going to arrive on Friday.

Call Squiggly whether or not you are going to arrive on Friday.

The first sentence is conditional. Call Squiggly if you are going to arrive on Friday means Aardvark is only expected to call if he is coming.

The second sentence is not conditional. Call Squiggly whether or not you are going to arrive on Friday means Aardvark is expected to call either way.

So to sum up, use whether when you have two discrete choices or mean "regardless of whether," and use if for conditional sentences.

Whether Versus Whether or Not

That last example is also a good lead in to the second part of the caller's question: when do you need an or not after whether? Why did I say, "Call Squiggly whether or not you are going to arrive on Friday"?

Often, the or not is just extra fluff and should be left off. In my first example, where I said, "Squiggly didn't know whether Aardvark would arrive on Friday," adding an or not wouldn't change the meaning or emphasis. Squiggly didn't know whether or not Aardvark would arrive on Friday means the same thing as Squiggly didn't know whether Aardvark would arrive on Friday. Or not is superfluous, so leave it out.

On the other hand, you need the full phrase whether or not when you mean "regardless of whether." It shows that there is equal emphasis on both options.

Call Squiggly regardless of whether you are going to arrive on Friday.

Call Squiggly whether or not you are going to arrive on Friday.

Finally, a listener wrote to say that her boss was driving her crazy by saying "rather or not" instead of "whether or not." So I'll add that "rather or not" is incorrect; whether is a conjunction and rather is an adverb, and they are not interchangeable. "Whether or not" is the correct way to show that there are two possibilities or you mean "regardless of whether."

Questions and comments for me go to feedback@quickanddirtytips.com, or you can submit them via Facebook or Twitter.

References

Bernstein, T.M. Do's Don'ts and Maybes of English Usage, Times Books: New York. 1977, p.237.

Garner, B. Garner's Modern American Usage, Oxford: Oxford University Press. 2003, p. 422.

if. American Heritage College Dictionary. Fourth Edition. Houghton Mifflin Company:Boston. 2007, p. 689.

rather. Dictionary.com. Dictionary.com Unabridged (v 1.1). Random House, Inc. http://dictionary.reference.com/browse/rather (accessed: May 23, 2008).

Kilian, C. "Rather? Whether?" Ask the English Teacher. May 26, 2006. http://crofsblogs.typepad.com/english/2006/05/rather_whether.html (accessed: May 23, 2008).

 

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