December hideg.

Because Hungarian often leaves out the verb to be in the present tense when the subject is third person and the predicate is a noun or an adjective.

So:

• December hideg. = December is cold.

This is completely normal Hungarian.

Compare:

• A víz hideg. = The water is cold.
• Ő tanár. = He/She is a teacher.

But in other persons or tenses, Hungarian does use forms of to be:

• Hideg vagyok. = I am cold.
• December hideg volt. = December was cold.

Yes—month names are normally lowercase in Hungarian.

So in the middle of a sentence, you would usually write:

• december

It is capitalized here only because it is the first word of the sentence.

Examples:

• December hideg.
• A december hideg szokott lenni.

So English and Hungarian are different here: English always capitalizes December, but Hungarian normally does not.

Here hideg is an adjective meaning cold.

It is being used as a predicate adjective, so it describes the subject December:

• December hideg. = December is cold.

You do not need any extra ending on hideg for this kind of sentence.

Also, Hungarian adjectives do not change to agree with the noun the way they do in some other languages. So hideg stays hideg.

Good question. The reason is that this sentence is not saying in December.

In December hideg, December is the subject of the sentence:

• December = the thing being talked about
• hideg = what is being said about it

So the meaning is:

• December is cold.

If you want to say in December, then yes, you need the ending -ban/-ben:

• decemberben = in December

For example:

• Decemberben hideg van. = It is cold in December.

So:

• December hideg. = December is cold.
• Decemberben hideg van. = It is cold in December.

Not here. In a simple sentence like this, December can appear without an article.

So:

• December hideg. is fine.

Hungarian often leaves out the article with names, including month names used this way.

That said, you may also see sentences where a december appears, especially in more descriptive or general statements:

• A december általában hideg. = December is usually cold.

So the short version is:

• No article is completely normal here.
• An article can also appear in some contexts, but it is not required in this simple sentence.

Yes, December hideg has a normal, straightforward word order:

• subject + predicate

So it literally works like:

• December
  • cold

If you say Hideg december, that will usually be understood as a noun phrase, not a full sentence:

• hideg december = a cold December

So the two are different:

• December hideg. = December is cold.
• Hideg december. = Cold December / a cold December (more like a phrase, or a poetic/elliptical sentence)

For a beginner, it is safest to remember:

• December hideg = full sentence
• hideg december = adjective + noun phrase

It is correct, but in everyday Hungarian, people will often prefer:

• Decemberben hideg van.

That version sounds more like a natural weather statement:

• It is cold in December.

The difference is small but useful:

• December hideg. = December is cold
  This describes December as a cold month.

• Decemberben hideg van. = It is cold in December
  This describes the weather in December.

So your sentence is grammatical and understandable, but a Hungarian speaker talking about climate or weather might often choose the -ben + hideg van pattern.

A simple guide is:

• December ≈ DE-tsem-ber
• hideg ≈ HI-deg

A few helpful pronunciation points:

• Hungarian stress is usually on the first syllable of each word.
• c in Hungarian is pronounced like ts.
• e is usually like the e in bed.
• g in hideg is a hard g, as in go.

So the whole sentence is roughly:

• DE-tsem-ber HI-deg

Not exact English pronunciation, of course, but that will get you close.