"If" and "Whether" - Is interrogative IF always replacable by WHETHER?

• There are a few verbs where I think "<verb> <interrogative clause>" in general is colloquial-but-acceptable, but where I think *"<verb> whether […]" in particular is ungrammatical; for example, consider "went to look if […]" vs. *"went to look whether […]", or "I can't think if I've […]" vs. *"I can't think whether I've […]".

  • In part this might be because whether is a bit more formal than if, so it doesn't work in these colloquialisms because the registers don't match; but I don't think that's a complete explanation, because I don't think the registers are so mismatched as to account for why the result sounds so ungrammatical (at least to me). (I'd welcome your thoughts on this.)

• This is probably cheating, but I don't think if can be replaced with whether in an echo question:

  "I'm wondering if she, uh . . ." He trailed off.
  *"Whether she what?"

  (Note that this isn't completely trivial, since echo questions do allow some kinds of substitutions; for example, if the first speaker had said "Hannah" instead of "she", it would still be fine for the second speaker to substitute "she". And I think that echo questions can usually replace the zero complementizer with that and vice versa. But even so, I describe this as "probably cheating" because it's probably more a fact about echo questions than a fact about if and whether.)

• You yourself pointed out in a comment above that something like "ask [if [whether …] …]" is clearly intelligible and grammatical, whereas something like "ask [whether [whether …] …]" is, um, not.

• This is obviously cheating, but hey: colloquial/dialectal if'n can never be replaced with whether'n. ;-)

This time I have read your post more carefully. I am guessing that the answer is "no". Whether can be used wherever interrogative if can. Conditional if has restricted distribution (lower type-wise frequency), but is more frequent token-wise. You see this a lot when looking at two words with similar meaning.

I have plotted below the top 50 hits for verb+if (red) (discarding instances of conditional if) and verb+whether (blue) from COCA to give you an idea.