I recall hearing
"Weiß ich nicht"
from the friends around when I stayed in Germany. Recently I started to take a German course to learn the language well. The lecturer told that there is a rule where you can swap the places of the 1st and the 3rd element of a sentence, as in
"Ich komme heute nach Hause" -> "Heute komme ich nach Hause".
I heuristically associated the two situations, however the usage I ask ("Weiß ich nicht") seems to be a question format. I might recalled the usage wrong, but I think I'm sure about what I heard. Is that a correct usage or a slang usage?
In addition to those two examples, Rammstein lyrics say:
"Ohne dich kann ich nicht sein".
Is that a regular sentence? It seems like it is not to me, I'd build the sentence as "Ohne dich, Ich kann nicht sein" or based on the swapping rule I mentioned, I might have gone for "Ich kann ohne dich nicht sein" but I'm absolutely not sure that this is a correct sentence. Can you explain me the situations of the three examples I gave?