Reggel fagy van.

Because Hungarian does not use a dummy subject like English it in weather expressions.

English says It is cold, It is raining, It is freezing.
Hungarian usually just says the condition itself:

• Hideg van = It is cold
• Esik = It is raining
• Fagy van = It is freezing / There is frost

So Reggel fagy van is completely normal without any separate word for it.

Van is the present-tense form of lenni, meaning to be.

In Hungarian, many weather and environmental conditions are expressed with a noun plus van:

• köd van = there is fog
• szél van = it is windy / there is wind
• fagy van = it is freezing / there is frost

So fagy van is a standard weather-type expression meaning that freezing conditions are present.

Here, fagy is best understood as a noun.

So:

• fagy van = there is frost / there is freezing weather

But fagy can also be a verb form from fagyni = to freeze:

• Éjjel fagy = It freezes at night

A useful shortcut is:

• if you see fagy van, think of fagy as a noun-like weather condition
• if you see fagy by itself as the main verb, then it is the verb freezes

Because this is a general weather expression, not a reference to a specific, identifiable frost.

Hungarian often leaves out the article in these set expressions:

• hideg van
• meleg van
• szél van
• fagy van

Using a here would usually sound wrong unless you were talking about some very specific frost in a special context.

Here reggel means in the morning.

It is functioning as a time expression, telling you when the freezing conditions happen.

So the structure is basically:

• Reggel = in the morning
• fagy van = it is freezing / there is frost

Together: In the morning, it is freezing or There is frost in the morning.

Because reggel already works as a built-in time adverb meaning in the morning.

Hungarian has several time words that often appear without extra endings:

• reggel = in the morning
• este = in the evening
• éjjel = at night
• délután = in the afternoon

So Reggel fagy van is the normal form.
A form like reggelen is not what you want here.

Because Hungarian often puts the time expression early in the sentence to set the time frame first.

So Reggel fagy van feels very natural and neutral: first you establish when, then you say what the situation is.

You can move things around in Hungarian, but the emphasis changes. For example:

• Reggel fagy van = neutral, in the morning there is frost
• Fagy van reggel = less neutral, with more emphasis on fagy van

For a learner, Reggel fagy van is a very good standard word order.

Yes, you can, but the meaning is structured differently.

• Reggel fagy van presents freezing as a state/condition
• Reggel fagy uses fagy as the verb freezes

So both can be possible, but:

• fagy van sounds like a weather condition
• fagy sounds more verbal, like the actual process of freezing

For many learners, fagy van is the safer pattern to remember because it matches other common weather expressions.

They are similar, but not identical.

• Reggel hideg van = It is cold in the morning
  This is more general. It could be cold without actually reaching freezing point.

• Reggel fagy van = It is freezing in the morning / There is frost in the morning
  This is more specific and stronger. It suggests actual frost or below-freezing conditions.

• Reggel fagyos az idő = The weather is frosty in the morning
  This is also natural, but it is a more descriptive way of saying it.

So fagy van is more specific than just hideg van.

You would say:

Reggel nincs fagy.

Hungarian does not usually say nem van here.
The negative of existential van is nincs.

So:

• fagy van = there is frost
• nincs fagy = there is no frost

This is a very useful pattern to learn:

• szél van → nincs szél
• köd van → nincs köd
• fagy van → nincs fagy

Usually you can keep the same word order and just use question intonation in speech, or a question mark in writing:

Reggel fagy van?

That means Is there frost in the morning? or Is it freezing in the morning?

Hungarian often forms simple yes-no questions without changing the word order very much. Intonation does a lot of the work.