"Wątpię, że on to zrobił."

Translation: I doubt that he did it.

[AnCatDubh]

‘I doubt if he did it’? Sounds way less natural to me than ‘I doubt he did it’ (rejected) or ‘I doubt that he did it’.

[Brambles07]

I agree; I've never heard anyone say 'I doubt if/that he did it'.

[Darlolass0]

To me all three sound OK, although "I doubt he did it" sounds most natural and "I doubt that he did it" sounds more correct than "if".

[Jellei]

All versions mentioned so far work.

"I doubt he did it" set as the best answer.

[DavidCampo986302]

Would "czy" be right instead of "że" here?

[Jellei]

Yes, but it seems to me that this could change the meaning a bit.

"że to zrobił", especially with specifying "że on to zrobił", can mean "that he is guilty of what they accuse him of doing". Like "I doubt he murdered her, I think he's innocent".

It can also mean I guess simply "I don't think that he did it", with 'it' that can be just 'cleaning his room'.

"czy" only has the second meaning in my opinion.

[Hugo647250]

I doubt that he did it sounds most natural to me.

[Jellei]

And 'that' is at least some equivalent of 'że'. Changed the default sentence.

[Czasami]

I doubt it he did that?