On the subject of whether to use "whether" or "if," the Glossary of Usage in the Harbrace 12th Edition says: "Use if in a state of condition [huh?] ; use whether as condition: I can't go if you drive; whether or not I go depends on who is driving.
In previous editions (I'm looking at the 6th and 4th), Harbrace said: "Some writers prefer whether to if after such verbs as say, learn, know, understand, doubt, especially when followed by or. As in, 'I did not know whether he would ride or walk.' "
Ruge Rules says flatly:
After such verbs as see, ask, doubt, wonder, question, use whether, not if.
Some rhetoricians offer a choice, but I [Ferdinand E. Ruge] do not think the use of whether instead of if is a matter of preference or desirability at all. You do not wonder a condition (if) but alternatives (whether); e.g., I wonder whether he is coming (or whether he is not coming).
There are much more important matters for you to be concerned with than the use of whether instead of if ; at the same time, I want you to observe the practice stated above.
I doubt whether the time is right to make a second attempt.
He asked me whether I had ever been there before.
I want to know whether you are ready.
Please see whether the water has been turned off.
I question whether the proposed course of action is desirable.
I will not try if the time is not right.
Anne Bagamery, business editor of the International Herald Tribune, adds the following words of common sense:
The journalists' shortcut is this: What makes the sentence clear?
"I didn't know whether he would walk or drive" is clear. (You knew he'd get there, but you didn't know which mode of transportation he would use.)
"I didn't know if he would walk or drive" is ambiguous. (What didn't you know? Whether he'd get there, or which mode of transportation?)