The conjunctions whether and if can be used
interchangeably in many cases.
Both whether and if
introduce a noun clause indicating uncertainty. The clause may appear
after a verb such as ask, doubt, know, learn, see or
wonder; and with some adjectives used with the negative be
such as not sure and not certain.
I asked Tom whether/if he would be at the meeting.
He
said he wasn’t sure whether/if he would be or not.
In a more formal style, however, whether is usually
preferred:
The Board of Directors did not decide whether they would
postpone the meeting.
The people in the town are ignorant; they
don’t know whether their mayor is a conservative or a
liberal..
To emphasize the fact that there are two alternatives, you may add
or not. (“If or not?as a phrase does not exist):
I don’t know whether they’re coming.
I don’t know if they’re
coming or not.
INCORRECT: I don’t know if or not they’re
coming.
Whether and if both introduce a clause concerning one
alternative, as in sentences (a) and (c) below. Whether or not,
whether?or not, and if?.or not introduce a clause with
both alternatives, as in sentences (b) and (d):
|
(a) |
We need to find out whether they can afford our
price. |
|
(b) |
We need to find out whether they can afford our price or
not. |
|
(c) |
We need to find out if they can afford our price. |
|
(d) |
We need to find out if they can afford our price or
not. |
Use whether (NOT
if) in these cases:
- When a yes/no question is a noun clause and is the subject,
whether, not if, must be used:
Whether Helen comes to the party or not doesn’t
matter.
INCORRECT: If Helen comes to the party or
not doesn’t matter.
- When the yes/no question is the object of a preposition:
It depends on whether or not she understands the
directions.
INCORRECT: It depends on if she understands the
directions or not.
- A few verbs such as discuss are usually followed by
whether, NOT if:
We discussed whether the meeting was going to be
important.
Use if (NOT whether) in this case:
To mean “in
the event that,?B> if (NOT whether) introduces an adverb
clause:
We’ll go on a picnic if it doesn’t rain.
If you lower the price, a lot more people will buy your
product.
If you love him, marry him!
Finally, since it's possible that there could be ambiguity with
if (if introduces conditional clauses as well as those
describing alternatives), The Oxford Dictionary of American Usage and
Style (by Bryan A. Garner, Oxford University Press,
2000) suggests this about if and whether:
“It’s good practice to distinguish between these words. Use
if for a conditional idea, whether for an alternative or
possibility. Thus, Let me know if you’ll be coming means
that I want to hear from you only if you’re coming. But Let me know
whether you’ll be coming means that I want to hear from you
about your plans, one way or the other.?/P>
So, while it is usually perfectly clear from the discourse whether
the speaker is referring to a conditional situation or a choice, a
careful speaker and certainly a careful writer would make a distinction
between if and whether for total accuracy.