never doubted that/if/whether

[joham]

He never doubted that/if/whether he would return.

Do all three conjunctions work for a negative sentence and without difference?

Thank you in advance.

 

[5jj]

We normally use 'that' with negative forms of 'doubt':

[FONT=&quot]"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 them:[/FONT][FONT=&quot]

1. I doubt if he was there.[/FONT][FONT=&quot]
2.I doubt that he was there.[/FONT]

[FONT=&quot]In [1] the uncertainty about his being 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.[/FONT]

[FONT=&quot]With absence of doubt we return to certainty, and THAT is now the only appropriate choice:[/FONT][FONT=&quot] 3. I don't doubt that he succeeded."[/FONT]


http://www.gramorak.com/Articles/If.pdf