A fűtés energiahatékony, hacsak nem hibás a termosztát.

In Hungarian, the present-tense verb to be is usually omitted in the 3rd person singular and plural when the predicate is a noun or adjective.

So:

• A fűtés energiahatékony. = The heating is energy-efficient.
• Hibás a termosztát. = The thermostat is faulty.

But in other tenses, or in 1st/2nd person, you do need the verb:

• A fűtés energiahatékony volt. = The heating was energy-efficient.
• Én fáradt vagyok. = I am tired.

Hacsak nem means unless.

It introduces an exception:

• A fűtés energiahatékony, hacsak nem hibás a termosztát. = The heating is energy-efficient, unless the thermostat is faulty.

It is best learned as a fixed expression.

Because in Hungarian, hacsak nem is the normal fixed way to say unless.

Even though nem literally means not, you should not translate the phrase word by word here. As a whole, hacsak nem functions like one conjunction meaning unless.

So:

• hacsak nem = unless

This is one of those expressions that is more natural to memorize as a unit.

A is the Hungarian definite article, meaning the.

It appears before consonant sounds:

• a fűtés = the heating
• a termosztát = the thermostat

Before vowel sounds, Hungarian uses az instead:

• az autó = the car

Hungarian often forms compound words much more readily than English does.

energiahatékony is made up of:

• energia = energy
• hatékony = efficient

Together, energiahatékony means energy-efficient.

This is very normal in Hungarian, where compound adjectives and nouns are common.

No. Fűtés is a noun, meaning heating or the heating system/heating process, depending on context.

It comes from the verb fűt = to heat.

So in this sentence:

• A fűtés energiahatékony. means The heating is energy-efficient.

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

• A termosztát hibás. = neutral, straightforward: The thermostat is faulty.
• Hibás a termosztát. = puts more focus on faulty.

Hungarian word order is flexible, and it often reflects what is being emphasized or presented as the important information. After hacsak nem, the version with hibás first sounds very natural.

So the given sentence highlights the possibility that the thermostat might specifically be faulty.

Hibás means faulty, defective, or wrong, depending on context.

In this sentence:

• hibás a termosztát = the thermostat is faulty

Related word:

• hiba = fault, error, problem

So hibás is an adjective built from the idea of having a fault.

Not if you want the meaning unless the thermostat is faulty.

• hacsak nem hibás a termosztát = unless the thermostat is faulty
• ha nem hibás a termosztát = if the thermostat is not faulty

Those mean different things.

So hacsak nem is the correct choice for unless.

They are in the nominative form, which is the basic dictionary form.

That is because they are functioning as the subject in simple predicate sentences:

• A fűtés energiahatékony.
• Hibás a termosztát.

Hungarian does not need any special case ending here.

You need it when the sentence is not in the present 3rd person with a nominal or adjectival predicate.

For example:

• A fűtés energiahatékony volt. = The heating was energy-efficient.
• A fűtés energiahatékony lesz. = The heating will be energy-efficient.
• Én energiahatékony vagyok. = grammatically possible, I am energy-efficient
• Te hibás vagy. = You are faulty / wrong

So the omission is limited; it is not true that Hungarian simply never uses to be.

Because hacsak nem introduces a subordinate clause, and Hungarian normally separates such clauses with a comma.

So the structure is:

• main clause: A fűtés energiahatékony
• subordinate clause: hacsak nem hibás a termosztát

That is why the comma is there.