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I know that to say things such as “I know that you dance well”, one can use the form “Ich weiß, dass…”. In this case, I believe the sentence could be translated as “Ich weiß, dass tanzt du gut”.

How do you form the opposite kind of construct, that you don’t know a thing? In English and French, both languages I speak, that becomes if and it makes me wonder if it is the same in German. What is, for instance, the correct way to say “I don’t know if I can come tomorrow” in German?

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is if even correct? dont you have to use whether? – Alex Mar 8 at 8:30
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@Alex Both if and whether are possible. – elena Mar 8 at 9:31
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Still a good remark, if you can use whether in English, it is most likely ob in German. – Carsten S Mar 8 at 9:40
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The correct translation of your sentence would be "Ich weiß, dass du gut tanzt". "dass" introduces a sub-clause where the verb needs to be at the end. – PMF Mar 8 at 11:28
    
@Alex Native German speakers are usually good at distinguishing if (wenn/falls) and whether (ob) in English, but bad at distinguishing if and when (wenn/sobald). – Crissov Mar 8 at 11:33
up vote 19 down vote accepted

First, your example would be translated as “Ich weiß, dass du gut tanzt” (verb in the end in a dass… sub clause).

If you want to express doubt about something, you put the nicht with the weiß, just like in English: “I do not know if I can come tomorrow” is “Ich weiß nicht, ob ich morgen kommen kann”. Like you assumed, if is used (ob in German), cause that sentence does not state a fact (like your first example with dass) but it expresses a possibility (if).

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The general form in German is

Ich weiß, dass ...

The opposite would be

Ich weiß nicht, ob ...


I'm going to elaborate on the comment I gave yesterday a bit to make clearer why wenn is not an option here:

According to the dictionaries, the English if can be translated (among others) as wenn, falls or ob in German, but just like it is not always correct in English to exhange if, when and whether, it is not correct in German to exchange falls, wenn and ob.

A rule of thumb is: In cases where you can use whether instead of if in English, you use ob in German. In other cases, wenn can be used, but mind the difference: there's a temporal wenn that specifies a point of time or something that occurs on a regular basis:

Wenn ich morgen in die Arbeit gehe, ...
(Immer) Wenn ich in die Arbeit gehe, ...

This would translate to the English when(ever). And then there's the wenn that expresses a possibility and can in most cases be replaced by falls.

Wenn/Falls es heute regnet, ...
Wenn/Falls ich heute abend noch auf die Party gehe, ...

This would translate to the English if/in case.

To add to the confusion, please keep also in mind that when can also translate to the German wann! So the following sentence is also correct

I don't know when I will be coming over.
Ich weiß nicht, wann ich vorbeikomme.

Stricly speaking, the part of the answer does not belong to this question, however...

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why ob rather than wenn? – Benubird Mar 8 at 16:05
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wenn is never an option in this case, even though if translates to wenn in some cases. However, if means wenn only if you can exchange it with falls: "if I go out tonight" translates to "wenn/falls ich heute Abend weggehe" whereas "when I go out tonight" translates to the temporal "wenn/zu dem Zeitpunkt wo ich heute Abend weggehe". "I don't know if/whether..." always translates to "... ob". wenn is simply not correct here. It never is when you can replace if by whether. – Thorsten Dittmar Mar 8 at 16:15

Agreeing with the above. The correct construct would be "Ich weiß nicht, ob..."

Elaborating though, good rules of thumb for ob/falls/wenn are:

ob -> "whether"

falls -> "in the case that" (think der Fall)

wenn -> other uses of "if" not covered above.

Sorry, would have ideally left this as a comment, but lack the necessary reputation.

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To negate most sentences in German, use nicht.

So, if "I know" translates to Ich weiß , then

"I don't know" translates to Ich weiß nicht.

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Thanks for trying to give my question an answer, but the part that I was struggling with wasn't the negation of the verb in the main clause, but how to introduce the subclause. – QuantumOmega Mar 9 at 15:44

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