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I think these two sentences have the same meaning, but which one should I use and why?

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The sentence "Ich weiß nicht" is colloquial but IMO not complete since it's missing an object. It should read "Ich weiß es nicht". es or Das (in your case) referring to the question (its subject) that someone has been asked for. – try-catch-finally Jun 21 '14 at 19:44
I don't think it's missing an object. It's missing an indirect question which is left out as it is totally clear from the context. – Max Ried Jun 23 '14 at 7:38
up vote 19 down vote accepted

The phrase

Ich weiß nicht.

is used in colloquial language to express uncertainty, doubt and disagreement, usually when asked for an opinion on something.

For example, I wouldn't be surprised to witness the following dialogue:

(Two girls in a shop)

A: Was hältst du von diesem Kleid?

B: Ich weiß nicht... - (thinks about it briefly) - Wenn du das nimmst, brauchst du jedenfalls auch passende Schuhe.

It is basically used as a filler in spoken language, much like a doubtful "Hm", to give the speaker a bit of time to think about how to phrase her doubts. Used like this, it is almost always followed by a pause and a more detailed explanation of the cause of the uncertainty.

The meaning depends a lot on context and tone of voice as well as facial expressions etc.

The phrase

Das weiß ich nicht.

means that the speaker actually does not know some fact.

A: Hat C seine Prüfung gestern bestanden?

B: Das weiß ich nicht, ich habe seither noch nicht mit ihm gesprochen.

As always when it comes to spoken language, the boundaries are a bit blurry and there are almost certainly regional differences.

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In a context where one may say "I don't know" in English without any further elaboration, I think "Weiss ich nicht" would be more common than "Ich weiss nicht." However, other phases like "Keine Ahnung" may well be even more common in practice.

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Hi and welcome to German Stack Exchange. While your answer’s details aren’t wrong, I think they just sli~ghtly miss the question. Not too bad in my opinion, but also not good enough to upvote. Do feel welcome and take a tour of the site! – Jan Jun 12 '15 at 16:50

1 Weiß nicht.

2 Weiß ich nicht.

3 Ich weiß nicht.

4 Ich weiß es nicht.

5 Das weiß ich nicht.

6 Das weiß ich wirklich nicht.

7 Das weiß ich leider nicht.

8 Keine Ahnung.

9 Ich habe keine Ahnung.

10 Ich habe keine Informationen.

11 Das ist mir nicht bekannt.

Jede dieser Formeln ist möglich. Ein kleines Kind könnte 1 benützen. Im lässigen Umgangston, unter Gleichgestellten, wird man 2-5 oder 8/9 hören. Wenn wir höflicher sein wollen, wird die Formel vollständiger sein. 10/11 ist mehr offizieller Ton.

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