Az érvényességi idő hosszú.

Az is the definite article, so it means the.

Hungarian has two forms of the definite article:

• a before a consonant
• az before a vowel

Because érvényességi begins with é, a vowel, the correct form is az: Az érvényességi idő.

This is a bit like English a/an in terms of sound choice, but remember that a/az means the, not a/an.

In Hungarian, the verb to be is usually omitted in the present tense, third person when the predicate is a noun or an adjective.

So:

• Az érvényességi idő hosszú. = The validity period is long.

There is no need to say van here.

But in other tenses, or with other persons, forms of to be do appear:

• Az érvényességi idő hosszú volt. = The validity period was long.
• Hosszú vagyok. = I am tall/long (grammatically showing that vagyok appears in 1st person)

So the missing is is completely normal.

Érvényességi idő is a common Hungarian expression meaning validity period, period of validity, or sometimes time until expiry, depending on context.

Literally, it is something like:

• érvényességi = relating to validity
• idő = time / period

Hungarian often builds phrases like this instead of using an of structure. So where English says period of validity, Hungarian uses a modifier + noun structure: érvényességi idő.

Because the two words do different jobs:

• érvényes = valid
• érvényesség = validity
• érvényességi = relating to validity

So:

• érvényes jegy = a valid ticket
• érvényességi idő = validity period

In other words, érvényes directly describes something as valid, while érvényességi is a relational adjective built from érvényesség. In this sentence, Hungarian needs the relational form because the phrase means the period of validity, not the valid time.

It does mean time, but in compounds and set expressions it can also mean period, duration, or term.

So in érvényességi idő, idő is not just abstract time in general. It means a defined span of time: a period.

That is why the whole phrase is best understood as validity period, not just validity time.

Here hosszú means long.

Hungarian uses hosszú for length and also for duration in many contexts. Since the subject is a time period, the natural meaning is long.

For tall, Hungarian normally uses magas, not hosszú.

So:

• hosszú út = a long road
• hosszú idő = a long time
• magas ember = a tall person

Yes, you could also say Hosszú az érvényességi idő.

The version you were given, Az érvényességi idő hosszú, is a neutral, straightforward statement: subject first, then the adjective.

If you move hosszú to the front:

• Hosszú az érvényességi idő.

that puts more emphasis on long. It can sound more contrastive or more like you are highlighting that particular fact.

So both are grammatical, but the word order affects emphasis and information structure.

There is no gender agreement in Hungarian, because Hungarian does not have grammatical gender like many Indo-European languages do.

In this sentence, the subject is singular, so hosszú stays singular too.

With a plural subject, the predicate adjective is often plural:

• Az érvényességi idők hosszúak. = The validity periods are long.

So the important point is:

• no masculine/feminine/neuter agreement
• singular here because the subject is singular

This is a basic nominative sentence with no special case endings.

• Az érvényességi idő is the subject
• hosszú is the predicate adjective

Nothing is being marked as an object, destination, source, possession by ending, and so on. It is just a simple statement: The validity period is long.