So here it is:
- What is this.
This is a window
Ez (egy) ablak. (Here egy can be omitted, but you can keep it in the sentence as well)
What is that?
That is a window.
- Az egy ablak. (I would not leave egy out in this case)
- Az? (Egy) ablak. (in this case it does not matter again, if you leave it out, or put it in the sentence)
There is no verb used in Hungarian in 3rd-person copula sentences. That means sentences of the fashion "[Thing] is [quality]". Or differently expressed: in sentences like these you do not use van or vannak. So for instance:
- A ház zöld. - The house is green.
- A kutyák nem nagyok. - The dogs are not big.
- Ő orvos. - He is a doctor.
- Ez a kórház. - This is the hospital.
You use a verb if the sentence is about a 1st or 2nd person:
- Orvos vagyok. - I am a doctor.
- Éhesek vagytok? - Are you hungry?
You use a verb if the sentence is in past tense:
- A macska így kicsi volt. - The cat was this small.
- Boltok voltak ezek az épületek. - These buildings were shops.
You use a verb if talking about where and/or when something is.
- Az áruház itt van. - The depratment store is here.
- Mikor van a születésnapod? - When is your birthday?
You use a verb when talking about existence (or more commonly non-existence). The examples I'd give here are a bit convoluted, so I'll skip them for now. You'll have a better idea once you get to possessive forms and how to express ownership. :)
Ah, you're mixing up word classes here. Nem negates what comes right after it, which is usually the verb: Nem vagyok jól - I am not well; A madarak nem repül - The birds do not fly.
If you move your nem anywhere else, it negates that word instead: Nem a madarak repül - Not the birds fly. Usually you'd wait then for an explanation what is flying instead.
Here we have a bit a special case because there is no verb. But we do not need one anyway. We're talking about what 'this' is: Ez nem ablak - This is not a window. Now if you move your nem in front of the ez, you get: Nem ez ablak - Not this is a window. Again you create an expectation to explain where the window is. Something like "but that" - hanem az.
Bottom line is, your suggestion is grammatically valid but expresses something different and sounds like only half a sentence.