Az önkéntesek csoportja nagy.

Hungarian has two forms of the definite article:

• a before a consonant sound
• az before a vowel sound

The next word is önkéntesek, which begins with the vowel ö, so Hungarian uses az:

• az önkéntesek = the volunteers

This is similar to how English uses a / an, but Hungarian does it with the definite article instead.

Önkéntesek means volunteers.

Its singular form is:

• önkéntes = volunteer

The ending -ek is a plural ending here:

• önkéntes → önkéntesek

Hungarian has several plural endings depending on vowel harmony and word shape, and -ek is the one used for this word.

Csoportja means his/her/its group or, in this kind of phrase, the group of ...

Here, Hungarian uses a possessive structure:

• az önkéntesek csoportja = literally the volunteers’ group
• natural English: the group of volunteers

So csoportja contains a possessive ending meaning its/their group, depending on context.

This is a very common Hungarian pattern:

• a diákok könyve = the students’ book
• a ház teteje = the roof of the house

Hungarian often expresses of X with a possessive construction rather than a separate word for of.

The -ja is a third-person singular possessive ending.

Base word:

• csoport = group

Possessed form:

• csoportja = his/her/its group

In this sentence, it is used in a noun phrase where the possessor is named:

• az önkéntesek csoportja = the group of the volunteers

The j often appears in possessive forms with certain nouns. You do not always just add -a or -e mechanically; some words take a linking j.

So this is something you usually learn as part of the noun’s possessive pattern:

• csoportja, not csoporta

Because the sentence is talking about one group, not several groups.

• önkéntesek = volunteers (plural possessor)
• csoportja = group (the thing possessed, singular)

So the meaning is:

• the volunteers’ group is big
• not the volunteers’ groups are big

The number of the possessor and the number of the possessed thing are separate.

Compare:

• a tanárok szobája = the teachers’ room → one room
• a tanárok szobái = the teachers’ rooms → several rooms

In Hungarian, the verb to be is usually omitted in the third person singular and plural in the present tense when talking about simple identification or description.

So:

• A ház nagy. = The house is big.
• Az önkéntesek csoportja nagy. = The group of volunteers is big.

But in other persons, the verb is expressed:

• Én nagy vagyok. = I am big.
• Te nagy vagy. = You are big.

So the missing is is completely normal here.

Yes, Hungarian word order is often more flexible than English, but this sentence uses a very basic neutral order:

• Az önkéntesek csoportja = subject
• nagy = predicate adjective

So the structure is simply:

• The group of volunteers
  • big

Because Hungarian omits is here, the adjective comes directly after the subject.

This is the most natural neutral version of the sentence.

Hungarian adjectives usually do not agree with the noun in number or gender the way they do in many Indo-European languages.

So:

• nagy ház = big house
• nagy házak = big houses

The adjective nagy stays the same.

Also, Hungarian has no grammatical gender, so there is no masculine/feminine/neuter agreement to worry about.

In your sentence, nagy stays unchanged even though the phrase contains a plural possessor (önkéntesek), because the adjective is describing csoportja and Hungarian adjectives do not change for that.

Not exactly.

• az önkéntesek csoportja = the group of volunteers
• az önkéntes csoport = the volunteer group

These are related, but not identical.

az önkéntesek csoportja

This emphasizes that the group belongs to or consists of the volunteers.

az önkéntes csoport

This is more like an attributive phrase: the volunteer group, a group characterized as volunteer-based.

In many situations they may refer to the same real-world thing, but grammatically they are different structures.

Hungarian usually does not use an apostrophe like English volunteers’, and it does not need a separate word like of here either.

Instead, Hungarian shows this relationship through structure:

• possessor first: az önkéntesek
• possessed noun with possessive ending: csoportja

So the possessive meaning is built into the grammar of the noun phrase.

A very literal breakdown is:

• az önkéntesek = the volunteers
• csoportja = their group

Together:

• az önkéntesek csoportja = the volunteers’ group / the group of volunteers

In some contexts, yes, but it would not mean exactly the same thing stylistically.

• Az önkéntesek csoportja nagy. = The group of volunteers is big.
• Önkéntesek csoportja nagy. = more like A group of volunteers is big or a less specific/general expression, depending on context

Hungarian often omits the article when speaking more generally or in certain stylistic situations, but with a specific identifiable group, az is the normal choice.

So for a straightforward sentence meaning the group of volunteers is big, the version with az is the best default.

Then the possessed noun would become plural:

• Az önkéntesek csoportjai nagyok. = The volunteers’ groups are big.

Changes:

• csoportja → csoportjai = groups
• nagy → nagyok = big in predicate position with a plural subject

This is a useful point: when the whole subject is plural, the predicate adjective can take a plural ending in this kind of sentence.

So your original sentence is singular because it refers to one group:

• Az önkéntesek csoportja nagy.