One of the core ideas of 'if' involves uncertainty. see: Gramorak.com
With some verbs expressing in themselves lack of certainty it appears that THAT and IF can both be used, with little practical difference in meaning between:
1. I doubt if he could do the job.
2. I doubt that he could do the job
In [1] the uncertainty about his ability to do the job there may be stronger than in [2]; only the context of the utterance can make this clear. The difference is not important for practical purposes.
With absence of doubt we return to certainty, and THAT is now the only appropriate choice:
3. I don't doubt that he could do the job
Thought I agree with most of what is said above I beg to differ when it comes to 'I doubt if.' In my view the core meaning of 'if' relates to contingency:
If you do this I'll do that. There is no contingency in sentences such as
I doubt if he could do the job. For me the only appropriate forms are:
- I doubt he could do the job.
- I doubt that he could do the job.
- I doubt whether he could do the job.
Ultimately, I know, English grammar is descriptive rather than prescriptive. This does not prevent us from considering some widely used structures to be incorrect. We do not accept
If he would do it I would do it too, for example, even though this structure is frequently used on both sides of the pond. Presumably this rejection is based on the grammatical clash that results with other aspects of grammar. I would argue that
I doubt if he could do the job is inconsistent in the same way as
I ain't got none, for example, and should not be taught in the classroom, especially following a lesson on the difference between 'if' and 'whether.' Without wishing to offend, I suggest that the use of 'if' in
I doubt if he could do the job is, at the very least, sloppy English, however widespread.
Does 'if' describe uncertainty? I don't think it does. The uncertainty in sentences such as
If the world ends tomorrow we'll all go to heaven comes from the information given in the sentence and does not stem from the use of 'if.' We generally think of such outcomes as uncertain. However, the general gist of
If all else fails you'll need a lawyer is one of certainty. In neither sentence could 'if' be construed as relating to uncertainty:
If the world ends tomorrow we'll all go to heaven is not equivalent to
In the uncertain event that the world ends tomorrow we'll all go to heaven. It is equivalent only to
In the event that the world ends tomorrow we'll all go to heaven.
If all else fails you'll need a lawyer is not equivalent to
In the uncertain event all else fails you'll need a lawyer. It is equivalent only to
In the event all else fails you'll need a lawyer.
If relates to
contingency and not to
uncertainty.