A melléklet a kérelemben van.

Because a is the Hungarian definite article, meaning the.

So the sentence breaks down like this:

• a melléklet = the attachment / the appendix / the enclosure
• a kérelemben = in the application / in the request

Hungarian uses the definite article very regularly, so if both nouns are definite, both can take a.

Also:

• a is used before consonant sounds
• az is used before vowel sounds

So you get:

• a melléklet
• a kérelem but
• az irat

Kérelemben consists of:

• kérelem = application, request, petition
• -ben = in

So:

• kérelemben = in the application/request

Hungarian often expresses meanings like in, on, under, from, to, etc. with endings attached directly to the noun rather than with separate prepositions.

Compare:

• házban = in the house
• dobozban = in the box
• kérelemben = in the application

This kind of ending is called a case ending. Here, -ban/-ben is the ending for in.

This is because of vowel harmony, a very important feature of Hungarian.

The ending for in has two forms:

• -ban
• -ben

Which one you use depends on the vowels in the word.

• kérelem contains front vowels, so it takes -ben
• for a back-vowel word, you would usually use -ban

Examples:

• kertben = in the garden
• székben = in the chair
• házban = in the house
• szobában = in the room

So kérelemben is the expected harmonic form.

Because Hungarian is an agglutinative language. That means grammatical meanings are often shown by adding endings directly to words.

In English, you say:

• in the application

In Hungarian, the same idea is usually expressed as:

• a kérelemben

So instead of a separate preposition like in, Hungarian uses a noun ending.

This is very normal in Hungarian:

• asztalon = on the table
• házból = out of the house
• iskolába = into the school
• kérelemben = in the application

That's a very common and very good question.

Hungarian often omits van in the present tense in sentences like:

• A melléklet fontos. = The attachment is important.
• A kérelem kész. = The application is ready.

But in sentences showing location or existence, van is normally used:

• A melléklet a kérelemben van. = The attachment is in the application.
• A könyv az asztalon van. = The book is on the table.

So the rule of thumb is:

• with identification or description, 3rd person present van is often omitted
• with location/existence, van is usually kept

That is why van appears here.

The basic structure is:

• A melléklet = topic / what we are talking about
• a kérelemben = location
• van = is

So:

• A melléklet a kérelemben van.
• literally: The attachment in the application is.

This is normal Hungarian word order for a neutral statement.

Hungarian word order is flexible, but changes in order often change emphasis rather than basic meaning.

For example:

• A melléklet a kérelemben van. = neutral: The attachment is in the application.
• A kérelemben van a melléklet. = emphasis more on where it is
• A melléklet van a kérelemben. = emphasis more on the attachment as the thing that is there

So the original sentence is a straightforward, neutral version.

Melléklet can mean things like:

• attachment
• appendix
• enclosure
• annex

The exact English word depends on context.

For example:

• in a document or official paperwork context, melléklet often means attachment or enclosure
• in a book or formal text, it might mean appendix
• in legal or administrative language, it can mean annex

So in this sentence, it most likely refers to some document or item included with the application/request.

This is the difference between in and into.

• -ban/-ben = in
• -ba/-be = into

So:

• a kérelemben van = it is in the application
• a kérelembe teszi = he/she puts it into the application

Examples:

• A levél a dobozban van. = The letter is in the box.
• A levelet a dobozba teszi. = He/she puts the letter into the box.

In your sentence, the meaning is location, not movement, so -ben is correct.

Yes. Even if the general meaning is already known, there can be a few natural English translations depending on context.

Possible translations include:

• The attachment is in the application.
• The enclosure is in the application.
• The appendix is in the application.
• The annex is in the application.

Also, kérelem can mean:

• application
• request
• sometimes petition, depending on context

So another possible translation could be:

• The attachment is in the request.

The grammar stays the same; only the best vocabulary choice changes with context.