Use of "if"/"whether" [duplicate]

Possible Duplicate:
When are “if” and “whether” equivalent?

I was told that one can use if in constructions like:

... if A, then ...

But that whether should be used when there is more than one option, like:

... whether A or B, then ...

Is it good practice or common to use whether when giving only one implicit or explicit option?

... whether A (or not), then ...

... whether A, then ...

No, it is not common practice to use whether with only one option.

According to the helpful Grammar Girl web site entry on whether vs. if:

The formal rule is to use if when you have a conditional sentence and whether when you are showing that two alternatives are possible.

This same page also has a section (with examples) about when "or not" is needed with the "whether". It says:

Often, the or not is just extra fluff and should be left off. ... 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.