Holnap a családdal ünnepeljük a testvérem születésnapját.

Családdal means with the family.

It is built from:

• család = family
• -val / -vel = with

So in theory you might expect családval, but Hungarian changes the v of -val/-vel to match the last consonant of the word before it. Since család ends in d, the result is:

• család + val → családdal

This kind of consonant doubling is very common in Hungarian:

• barát → baráttal = with a friend
• kéz → kézzel = with a hand

Hungarian uses the definite article a / az more regularly than English in many cases.

So a családdal is literally with the family, but in natural English it may simply mean with the family in the sense of with my/our family, depending on context.

In this sentence, a családdal sounds natural because it refers to a specific, understood family group.

Because Hungarian verbs have two main present-tense conjugations:

• indefinite conjugation
• definite conjugation

Here the object is definite and specific:

• a testvérem születésnapját = my sibling’s birthday

So Hungarian uses the definite form of the verb:

• ünnepeljük = we celebrate it / we are celebrating it

Compare:

• Születésnapot ünnepelünk. = We are celebrating a birthday.
  → indefinite object, so ünnepelünk
• A születésnapot ünnepeljük. = We are celebrating the birthday.
  → definite object, so ünnepeljük

Because Hungarian verbs usually show the subject in their ending, so subject pronouns are often left out unless they are needed for emphasis.

• ünnepeljük already tells you:
  • we
  • present tense
  • definite object

So mi = we is unnecessary here.

If you added mi, it would sound more emphatic:

• Mi ünnepeljük... = We are the ones celebrating...

Testvérem means my sibling or my brother/sister.

It breaks down like this:

• testvér = sibling
• -em = my

So:

• testvérem = my sibling

Important: Hungarian testvér does not tell you whether the person is male or female. It can mean:

• my brother
• my sister
• my sibling

You only know from context if needed.

Hungarian usually expresses possession with endings rather than with a separate apostrophe structure like English.

This part means my sibling’s birthday:

• testvérem = my sibling
• születésnapja = his/her birthday

So literally the structure is closer to:

• my sibling + birthday-his/hers

That is how Hungarian normally builds possessive phrases.

The possessor comes first:

• testvérem születésnapja = my sibling’s birthday

Because this noun is the direct object of the verb ünnepeljük.

Start with:

• születésnap = birthday
• születésnapja = his/her birthday

Then add the accusative ending -t because it is what is being celebrated:

• születésnapja → születésnapját

So:

• születésnapja = his/her birthday
• születésnapját = his/her birthday, as a direct object

In the full phrase:

• a testvérem születésnapját = my sibling’s birthday

Good question. Hungarian has two common possessive patterns.

A very common neutral pattern is:

• testvérem születésnapja = my sibling’s birthday

Another pattern uses -nak/-nek on the possessor:

• a testvéremnek a születésnapja

Both can mean my sibling’s birthday, but the version without -nak/-nek is very common and compact.

So in this sentence, testvérem születésnapját is a normal, natural possessive structure.

Because the whole noun phrase is definite:

• a testvérem születésnapját

Hungarian often puts the article at the beginning of the whole possessive phrase. So even though testvérem already means my sibling, the phrase can still begin with a.

This is normal in Hungarian:

• a barátom autója = my friend’s car
• a testvérem születésnapja = my sibling’s birthday

English does not work the same way, so this often feels strange to learners at first.

Yes. Hungarian word order is quite flexible, and changing it usually changes emphasis more than core meaning.

This sentence:

• Holnap a családdal ünnepeljük a testvérem születésnapját.

is a natural way to say:

• Tomorrow, we’re celebrating my sibling’s birthday with the family.

But other orders are possible, for example:

• A testvérem születésnapját holnap ünnepeljük a családdal.
• A családdal holnap ünnepeljük a testvérem születésnapját.

These versions shift attention to different parts:

• holnap = tomorrow
• a családdal = with the family
• a testvérem születésnapját = my sibling’s birthday

So the meaning stays basically the same, but the focus changes.

Holnap means tomorrow.

It is already a complete time adverb, so it does not need any extra ending here.

Hungarian often puts time expressions near the beginning of the sentence:

• Holnap a családdal ünnepeljük...

That is very natural, just like English often begins with:

• Tomorrow, ...

Yes, that is a helpful rough breakdown.

Word by word:

• Holnap = tomorrow
• a családdal = with the family
• ünnepeljük = we celebrate / we are celebrating it
• a testvérem = my sibling
• születésnapját = his/her birthday, as the object

A more structural gloss would be:

• Tomorrow, with the family, we are celebrating my sibling’s birthday.

So although the English translation sounds simple, the Hungarian sentence packs a lot of grammar into endings.