A gázszámla nagy a fűtésszezonban.

In Hungarian, the verb to be is usually left out in the present tense when the subject is third person and the predicate is a noun or adjective.

So:

• A gázszámla nagy. = The gas bill is big/high.
• Not: A gázszámla van nagy. That would be wrong here.

But Hungarian does use forms of van in other situations, for example:

• A gázszámla nagy volt. = The gas bill was high.
• A gázszámla nagy lesz. = The gas bill will be high.

So the missing is is completely normal.

A is the Hungarian definite article, meaning the.

So in this sentence:

• A gázszámla = the gas bill
• a fűtésszezonban = in the heating season

Hungarian uses the definite article very regularly, often in places where English also uses the.

There are two forms of the definite article:

• a before consonant sounds
• az before vowel sounds

For example:

• a gázszámla
• a fűtésszezon
• az autó

Since both gázszámla and fűtésszezonban begin with consonant sounds, a is used.

Yes, gázszámla is one word, and it is a compound noun:

• gáz = gas
• számla = bill

So literally it is gas bill.

Hungarian often combines words into one compound where English would use two separate words. This is very common:

• gázszámla = gas bill
• villanyszámla = electricity bill
• fűtésszezon = heating season

So if you see a long Hungarian noun, it is often made of smaller familiar parts.

Yes, nagy literally means big or large, but Hungarian often uses it where English would more naturally say high.

So:

• A gázszámla nagy. literally: The gas bill is big.
• natural English meaning: The gas bill is high.

This kind of mismatch is normal between languages. Hungarian may use a more physical adjective where English prefers a more abstract one.

Fűtésszezonban means in the heating season.

It breaks down like this:

• fűtés = heating
• szezon = season
• -ban = in

So:

• fűtésszezon = heating season
• fűtésszezonban = in the heating season

The ending -ban/-ben is called the inessive suffix, and it usually means in or inside.

Examples:

• a házban = in the house
• a városban = in the city
• a fűtésszezonban = in the heating season

This is because of vowel harmony, an important feature of Hungarian.

Hungarian suffixes often change form depending on the vowels in the word. A simplified version is:

• back vowels → -ban
• front vowels → -ben

In fűtésszezon, the word contains the back vowel o, so the suffix is -ban:

• fűtésszezonban

Compare:

• házban = in the house
• kertben = in the garden

Vowel harmony is something you will see constantly in Hungarian suffixes.

That is the most neutral, natural word order here.

Hungarian word order is flexible, but it is not random. It often reflects topic, focus, and emphasis.

The neutral structure here is:

• A gázszámla = topic
• nagy = predicate/new information
• a fűtésszezonban = time phrase

So the sentence simply states a general fact:
The gas bill is high during the heating season.

You can move parts around for emphasis, for example:

• A fűtésszezonban nagy a gázszámla.

This also means the same thing, but now during the heating season is more prominent.

It most naturally sounds like a general statement:
The gas bill is high in the heating season / Gas bills are high during the heating season.

Hungarian often uses the definite article in statements that English might express more generally.

So although A gázszámla literally looks like the gas bill, in context it can express a general idea about that kind of bill.

If the speaker were talking about one particular bill in a particular situation, the broader context would usually make that clear.

Yes, magas may also appear in contexts involving prices, costs, levels, or amounts, and it can mean high.

However, with something like a bill, nagy is very natural and common. It gives the idea that the bill is large in amount.

So:

• A gázszámla nagy a fűtésszezonban. = natural
• A gázszámla magas a fűtésszezonban. = possible, but may sound a bit less everyday depending on context

For learners, nagy is the safer choice in this sentence.