الملابس الرسمية في قاعة الزفاف.

It is most naturally understood as a phrase, not a complete sentence.

Literally, it is something like:

• the formal clothes / formal attire
• in the wedding hall

In context, this could work as a label, heading, sign, or caption.

If you wanted a full sentence in Arabic, you might say something like:

• تُطلَب الملابس الرسمية في قاعة الزفاف — Formal attire is required in the wedding hall.
• يُشترط ارتداء الملابس الرسمية في قاعة الزفاف — Wearing formal attire is required in the wedding hall.

Word by word:

• الملابس = the clothes / clothing
• الرسمية = formal / official
• في = in
• قاعة = hall
• الزفاف = the wedding

So the whole phrase refers to formal attire in the wedding hall.

Because in Arabic, adjectives normally come after the noun they describe.

So:

• ملابس رسمية = formal clothes
• الملابس الرسمية = the formal clothes / the formal attire

This is the normal Arabic order:

• noun + adjective

Unlike English, Arabic does not usually say formal clothes with the adjective first.

Because Arabic adjectives agree with the noun in definiteness.

Here:

• الملابس is definite because it has الـ
• so the adjective must also be definite: الرسمية

Compare:

• ملابس رسمية = formal clothes
• الملابس الرسمية = the formal clothes / the formal attire

So if the noun has الـ, the adjective usually does too.

This is a very common Arabic pattern.

الملابس is a non-human plural, and in Modern Standard Arabic, non-human plurals are very often treated grammatically like feminine singular when they take adjectives.

So:

• الملابس = plural in meaning
• but the adjective is feminine singular: الرسمية

That is why you get:

• الملابس الرسمية

and not a human-style plural adjective.

The same pattern appears in many examples:

• الكتب الجديدة = the new books
• السيارات السريعة = the fast cars

Even though the nouns are plural, the adjective is feminine singular because they are non-human plurals.

Because this is an iḍāfa construction, often called the construct phrase.

قاعة الزفاف literally means:

• hall of the wedding

In Arabic, possession or close noun relationships are often expressed by putting two nouns together:

• قاعة = hall
• الزفاف = the wedding

So:

• قاعة الزفاف = the wedding hall

Important rule: In an iḍāfa, the first noun usually does not take الـ, even when the whole phrase is definite.

So Arabic says:

• قاعة الزفاف

not:

• القاعة الزفاف

Because Arabic often expresses of relationships without a separate word.

English says:

• the hall of the wedding
• or more naturally, the wedding hall

Arabic uses the iḍāfa pattern instead:

• قاعة الزفاف

So the meaning of of is built into the structure itself.

Other examples:

• باب البيت = the door of the house / the house door
• كتاب الطالب = the student’s book / the book of the student

Yes, but the two words are not exactly identical.

• الزفاف usually refers more to the wedding celebration, wedding event, or ceremony
• الزواج usually refers more to marriage as a state or institution

So in قاعة الزفاف, الزفاف is very natural because we are talking about a wedding venue or event space.

A قاعة الزواج would sound less natural in most contexts.

A careful pronunciation is roughly:

al-malābisu r-rasmiyyatu fī qāʿati z-zifāfi

In normal pause-based pronunciation, learners often say something closer to:

al-malābis ar-rasmiyya fī qāʿat az-zifāf

A few useful pronunciation notes:

• الملابس = al-malābis
• الرسمية is pronounced ar-rasmiyya, not al-rasmiyya, because ر is a sun letter
• الزفاف is pronounced az-zifāf, not al-zifāf, because ز is also a sun letter
• قاعة contains ع, which is a deep throat consonant
• In قاعة الزفاف, the ة in قاعة is pronounced like t because it is followed by another noun in an iḍāfa: qāʿat az-zifāf

Because the final ة (tāʾ marbūṭa) is often pronounced as -a / -ah when the word stands alone, but as -t when the word is followed grammatically by another word in certain constructions, including iḍāfa.

So:

• alone: قاعة ≈ qāʿa
• in iḍāfa: قاعة الزفاف ≈ qāʿat az-zifāf

This is completely normal and very common in Arabic.

If fully vocalized in formal grammar, it would be:

الملابسُ الرسميةُ في قاعةِ الزفافِ

Why?

• الملابسُ: nominative here if it is the main noun phrase
• الرسميةُ: adjective matching it
• في makes the following noun genitive
• so قاعةِ
• and in the iḍāfa, الزفافِ is also genitive

So the grammatical ending pattern is:

• الملابسُ الرسميةُ
• في قاعةِ الزفافِ

In everyday reading and speech, these case endings are often not pronounced fully, but they are useful to know in MSA.

Yes.

• ملابس رسمية = formal clothes / formal attire
• الملابس الرسمية = the formal clothes / the formal attire

The version without الـ is indefinite and more general. The version with الـ is definite.

Which one is better depends on context:

• as a general category: ملابس رسمية
• as a specific known requirement or label: الملابس الرسمية

Both are grammatically correct.