Az országgyűlés határozata érvényes.

Hungarian has two forms of the definite article:

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

Since országgyűlés starts with the vowel o, the correct form is az országgyűlés.

Országgyűlés means National Assembly or Parliament.

It is a compound word:

• ország = country
• gyűlés = assembly, meeting

So the literal sense is something like assembly of the country.

Because határozata contains a possessive suffix.

• határozat = resolution, decision
• határozata = its resolution / his/her/its decision

In this sentence, the possessor is az országgyűlés. So:

• az országgyűlés határozata = the parliament's resolution / the resolution of parliament

Hungarian often shows possession by adding a suffix to the possessed noun.

Hungarian usually does not need a separate word like of in this kind of phrase.

Instead, it uses this pattern:

• possessor + possessed noun with possessive suffix

So:

• az országgyűlés határozata

literally works like:

• the parliament its-resolution

but in natural English it means:

• the parliament's resolution
• the resolution of parliament

So the of relationship is built into the structure, not shown by a separate word.

It is in the basic form; there is no special genitive case ending here.

English uses of or 's:

• the parliament's resolution

Hungarian often leaves the possessor in its plain form and marks the possessed noun instead:

• országgyűlés határozata

So the ownership relationship is shown mainly by határozata, not by changing országgyűlés.

In a normal Hungarian possessive construction, the article usually appears before the possessor, not separately before the possessed noun.

So:

• Az országgyűlés határozata = The parliament's resolution

This whole phrase is already definite, so Hungarian does not need another article before határozata here.

A more expanded structure is also possible in some contexts:

• Az országgyűlésnek a határozata

But the shorter version in your sentence is very normal.

Hungarian often omits the verb "to be" in the present tense, third person when the predicate is a noun or adjective.

So:

• Az országgyűlés határozata érvényes.

literally looks like:

• The parliament's resolution valid.

but it means:

• The parliament's resolution is valid.

This is completely normal Hungarian grammar.

If the sentence were in the past or future, or in first/second person, a form of van would appear.

Érvényes is an adjective here, used as a predicate adjective.

It means things like:

• valid
• effective
• in force

So in this sentence it describes the resolution:

• The parliament's resolution is valid.

Yes, by itself határozata can mean his/her/its resolution.

Hungarian third-person possessive forms do not show gender, so the ending does not tell you his, her, or its.

In this sentence, the owner is made clear by the noun before it:

• az országgyűlés határozata = the parliament's resolution

So the context tells you whose resolution it is.

Yes, Érvényes az országgyűlés határozata is possible, but the emphasis changes.

The original order:

• Az országgyűlés határozata érvényes.

is a neutral statement:

• As for the parliament's resolution, it is valid.

Starting with Érvényes puts more focus on the idea of validity:

• Valid is the parliament's resolution / The parliament's resolution is indeed valid

Hungarian word order is flexible, and it often reflects focus and emphasis, not just basic grammar.

It depends on how it is being used.

• Országgyűlés with a capital letter is common when referring to the specific Hungarian Parliament as an official institution.
• országgyűlés with a lowercase letter can appear when the word is treated more like a common noun or depending on the style of the source.

So learners may see both, but if you mean the official body of Hungary, Országgyűlés is very common.

This is the third-person singular possessive ending.

For many nouns, Hungarian adds:

• -a / -e or
• -ja / -je

depending on the shape of the word.

Here:

• határozat → határozata

So the noun gets the meaning its resolution.

The exact form of the ending depends on Hungarian sound patterns and noun type, so it is something learners usually pick up gradually through examples.