هذا حفل زفاف.

Because Arabic often leaves out the present-tense verb to be.

So هذا حفل زفاف literally looks like this wedding ceremony, but in natural English it means This is a wedding ceremony.

This type of sentence is called a nominal sentence. In the present tense, Arabic normally does not use a separate word for is/am/are.

• هذا = this
• حفل زفاف = a wedding ceremony

If you wanted a past-tense version, Arabic would use a verb, such as كان.

Because حفل is a masculine singular noun, and هذا is the demonstrative used with masculine singular nouns.

• هذا = this (masculine singular)
• هذه = this (feminine singular)

Since the thing being identified is حفل (ceremony/celebration), the masculine form هذا is the correct choice.

They form an iḍāfa construction, often called a genitive construction or noun-noun construction.

So:

• حفل = ceremony / celebration
• زفاف = wedding
• حفل زفاف = wedding ceremony

In other words, the second noun specifies the first one. It is not an adjective here. Arabic often expresses ideas like wedding ceremony, school book, house door, etc. by putting two nouns together in this way.

Because the phrase is indefinite here: a wedding ceremony, not the wedding ceremony.

So:

• حفل زفاف = a wedding ceremony
• حفل الزفاف = the wedding ceremony

Also, in an iḍāfa construction, the first noun normally does not take ال. Definiteness is often shown by the second noun.

That is why حفل الزفاف can mean the wedding ceremony, even though حفل itself does not have ال.

A fully vowelled version would be:

هَذَا حَفْلُ زِفَافٍ

Here is what is happening:

• هَذَا = this
• حَفْلُ = nominative, because it is the predicate of the sentence
• زِفَافٍ = genitive, because it is the second noun in the iḍāfa

A useful detail: if حفل were by itself and indefinite, it could be حفلٌ. But in iḍāfa, the first noun loses its tanwīn, so it becomes حفلُ زفافٍ, not حفلٌ زفافٍ.

A careful pronunciation is roughly:

hādhā ḥaflu zifāfin

In normal pause, many learners will hear something closer to:

hādhā ḥafl zifāf

A few sound notes:

• ذ in هذا sounds like th in this
• ح in حفل is a deep, breathy sound that does not exist in English
• زفاف has a long ā sound in the second syllable

In this exact form, it is naturally understood as:

This is a wedding ceremony.

That is because حفل زفاف is indefinite.

In Arabic, a pattern like:

هذا + indefinite noun

usually gives a full sentence meaning this is a...

If you want this wedding ceremony as a noun phrase, make it definite, for example:

هذا حفلُ الزفافِ

That makes حفل الزفاف a definite phrase: the wedding ceremony, so the whole expression can mean this wedding ceremony.

That is a very common Arabic pattern.

Even though the whole phrase is indefinite, حفل is the first noun in an iḍāfa construction, and the first noun in iḍāfa does not take tanwīn.

So you get:

• not حفلٌ زفافٍ
• but حفلُ زفافٍ

This is something English speakers often notice early, because it feels unusual at first. In Arabic, indefiniteness of the whole phrase does not mean every word must show tanwīn.