5

Both seems to be used often. Zamenhof uzis ambaŭ:

  • neniu dubos, ke konstanta regulo estas mil fojojn pli bona

  • mi dubas, ĉu troviĝos iu

Is there now a difference in meaning, or is one better than the other?

10

Only use the word ĉu here when you want to emphasize uncertainty. Then it means something like sin demandi ĉu.

Neniu dubas, ke tio estas fakto. No one doubts that it is a fact.

Li dubis ke mi aĝis nur ok. He doubted that I was only eight. (He suspected not.)

Li dubis ĉu mi aĝis nur ok. He had doubts about me being eight. (He continued to wonder about it.)

I base this on Zamenhof's Lingva Respondo 21-a ("Pri la vorto ĉu").

2

I agree with Andrew's answer and would like to point out that a verbatim translation to English or German gets almost the same meaning across, even if leading to non-idiomatic sentences.

Keeping the following translation in mind can thus help to remember which one means what:

  • ke → en:that, de:dass
  • ĉu → en:whether (or en:if), de:ob

Li dubis, ke mi aĝis nur ok.

He doubted that I was only eight.

Er bezeifelte, dass ich erst acht war.


Li dubis, ĉu mi aĝis nur ok.

He doubted whether I was only eight.

or more colloquially:

He doubted if I was only eight.

(Both a bit strange. It'd probably be more natural to use the verb "to wonder" in English, but still with whether or if. It conveys a slightly different meaning though.)

Er bezeifelte, ob ich erst acht war.

(Same here. Er fragte sich, ob ich erst acht war. would be more natural, but may imply that he kept that doubt to himself, while de:bezweifeln –like eo:dubi, en:doubt and en:wonder– leaves open whether the doubt was uttered or not.

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