Nincs huzat, hacsak nem rossz a nyílászáró.

In Hungarian, the negative form of van (to be / there is) is irregular:

• van = there is / is
• nincs = there is not / is not

So:

• Van huzat. = There is a draft.
• Nincs huzat. = There is no draft.

Using nem van would be ungrammatical in standard Hungarian.

Huzat means draft or draught: unwanted moving air coming in through a window, door, or some gap.

So Nincs huzat means there is no draft in the room or building.

Hacsak nem is a fixed expression meaning unless.

So:

• Nincs huzat, hacsak nem rossz a nyílászáró. = There is no draft, unless the window/door fitting is bad.

Even though nem normally means not, in hacsak nem you should usually learn the whole phrase as one unit meaning unless.

Because hacsak nem is an idiomatic Hungarian conjunction. It does not translate word-for-word very neatly.

You can think of it as something like:

• except if it is the case that...
• unless...

So the nem is part of the established expression, not a separate English-style negation you need to interpret literally each time.

Nyílászáró is a fairly technical or general term for a window, door, or similar closing structure in a wall opening.

Literally, it is made from:

• nyílás = opening
• záró = closing thing / closer

In everyday English, depending on context, it may be best understood as:

• window
• door
• window or door
• frame / fitting / joinery in a broader building sense

In normal everyday Hungarian, people often say ablak or ajtó instead if they want to be specific.

Hungarian often uses the singular when speaking about something in a general or representative way.

Here, a nyílászáró can mean:

• the relevant window/door fitting
• whichever opening closure is involved
• the door or window in question

So singular is perfectly natural. English also sometimes does this: unless the window is faulty, even if the general idea could apply to any window or door.

Both are possible, but the word order changes the emphasis.

• Rossz a nyílászáró. = The window/door fitting is bad.
  This is a very natural neutral way when the important new information is rossz.

• A nyílászáró rossz. = The window/door fitting is bad.
  This can sound a bit more like as for the window/door fitting, it is bad, or put more focus on the subject.

In this sentence, rossz a nyílászáró fits well because the condition being introduced is the possible problem: unless the fitting is faulty.

It is in the nominative case, which in Hungarian usually has no ending.

Here a nyílászáró is the subject of rossz:

• a nyílászáró = the window/door fitting
• rossz = bad

So literally: the window/door fitting is bad.

There is no visible verb because in Hungarian, in the present tense, third person singular, the verb to be is usually omitted in simple descriptive sentences.

So:

• A nyílászáró rossz. = The window/door fitting is bad.
• not A nyílászáró rossz van or anything like that

This is very normal Hungarian grammar.

Compare:

• Én beteg vagyok. = I am sick.
• Ő beteg. = He/she is sick.

The verb appears with I and you, but is usually omitted with he/she/it in the present tense.

Yes. Nincs and nincsen both exist.

• nincs is the more common everyday form
• nincsen is a fuller variant, sometimes sounding slightly more formal, emphatic, or stylistically different depending on context

So:

• Nincs huzat.
• Nincsen huzat.

Both are correct.

Yes, but nyílászáró sounds somewhat technical, administrative, or building-related.

A more everyday version might be something like:

• Nincs huzat, hacsak nem rossz az ablak vagy az ajtó.

The original sentence is still perfectly natural, especially in contexts like construction, repairs, insulation, real estate, or product descriptions.

Yes.

• Van huzat. = There is a draft.
• Nincs huzat. = There is no draft.

This is the normal existential pattern in Hungarian:

• van for existence
• nincs for negative existence

So the first half of the sentence is a standard there is / there isn’t construction.