"Is that the park?"
Translation:Ist das der Park?
I have still yet to understand the difference between "akkusative" and the bloody others
If you say something 'IS' something, then neither one is acting upon the other (and neither is receiving from the other). Accusative is used when one thing does something to another. In this case, 'that' IS 'the park'. That is not doing anything to the park - that is the park. Therefore it is nominative.
Another way of looking at it, the 'park' and 'that' are actually referring to the same thing. You could swap them round ('the park' is 'that') and it would still keep the meaning. One can't be doing anything to the other, because there is no 'other'. It is like saying x=x. One thing on its own is nominative, and here we still only have one thing, just two ways of referring to it.