Pénteken nem a városban vacsorázunk, hanem otthon.

Because Hungarian often adds a suffix to days of the week when it means on that day.

• péntek = Friday
• pénteken = on Friday

The ending -en here is one of the forms used for this time expression. With days, Hungarian usually does not need a separate word for on the way English does.

Examples:

• Hétfőn dolgozom. = I work on Monday.
• Szerdán utazunk. = We travel on Wednesday.
• Pénteken otthon maradunk. = We stay home on Friday.

This is a very common Hungarian pattern meaning not ..., but rather ... or not ..., but ....

In your sentence:

• nem a városban = not in the city
• hanem otthon = but at home

So the structure is:

• nem X, hanem Y = not X, but Y

More examples:

• Nem te, hanem én. = Not you, but me.
• Nem ma, hanem holnap. = Not today, but tomorrow.
• Nem kávét iszom, hanem teát. = I’m not drinking coffee, but tea.

This pattern is especially used when correcting or contrasting information.

Because a is the definite article, meaning the.

• város = city
• városban = in a city / in city
• a városban = in the city

In this sentence, the speaker is talking about a specific, understood place, so a városban is natural.

Also, Hungarian often uses the definite article in places where English may say something more idiomatic like in town. So depending on context, a városban could sometimes match English in town, even though it literally looks like in the city.

The ending -ban means in.

• város = city
• városban = in the city / in a city

Hungarian uses suffixes instead of separate prepositions in many cases.

Some similar examples:

• házban = in the house
• iskolában = in the school
• boltban = in the shop

The form -ban / -ben follows vowel harmony:

• -ban after back vowels
• -ben after front vowels

So:

• városban
• kertben

Because otthon is already a complete adverb meaning at home or home in this kind of sentence.

So Hungarian says:

• otthon = at home

not:

• otthonban

This is similar to how English says at home, not in home.

Compare:

• Itthon vagyok. = I am at home / I’m home.
• Otthon vacsorázunk. = We have dinner at home.

So a városban needs the location suffix, but otthon is already a location word by itself.

vacsorázunk means we have dinner or we are eating dinner.

It comes from the verb vacsorázik = to have dinner / to dine.

The ending -unk shows:

• 1st person plural
• so: we

That means the subject mi = we does not have to be stated, because the verb already tells you who the subject is.

So:

• vacsorázom = I have dinner
• vacsorázol = you have dinner
• vacsorázik = he/she has dinner
• vacsorázunk = we have dinner

Hungarian often leaves out the subject pronoun unless it is needed for emphasis.

Because Hungarian verbs usually show the subject clearly by their endings.

In vacsorázunk, the ending -unk already means we, so adding mi is unnecessary unless you want emphasis.

So:

• Vacsorázunk. = We are having dinner.
• Mi vacsorázunk. = We are having dinner.
  This version sounds more emphatic, like we are the ones having dinner.

This is very normal in Hungarian.

Because Hungarian word order often shows focus and contrast, not just basic sentence roles.

Here, the important contrast is between:

• a városban = in the city
• otthon = at home

In Hungarian, the contrasted element often comes right before the verb.

So:

• nem a városban vacsorázunk
  literally something like it is not in the city that we are having dinner
• hanem otthon
  but at home

This word order highlights the correction: the place is the important thing being contrasted.

If you changed the order, the emphasis could change or the sentence could sound less natural in this context.

Because Hungarian can leave out repeated words when they are understood from the first part of the sentence.

So:

• Pénteken nem a városban vacsorázunk, hanem otthon.

really means:

• On Friday we’re not having dinner in the city, but at home.

The verb vacsorázunk does not need to be repeated after hanem, because it is already clear.

A fuller version would be possible in some contexts, but it is usually unnecessary:

• Pénteken nem a városban vacsorázunk, hanem otthon vacsorázunk.

That sounds more repetitive.

It can sometimes correspond to either one, depending on context.

Literally:

• a városban = in the city

But in natural translation, English might say:

• in town
• downtown
• in the city

The exact English version depends on what the speaker means in the situation.

So learners should remember:

• Hungarian form: a városban
• natural English meaning: may vary by context

Yes. vacsorázunk is present tense.

Hungarian often uses the present tense for scheduled or planned future events, just like English sometimes does.

So:

• Pénteken vacsorázunk.
  can mean
• We’re having dinner on Friday
  or
• We’ll have dinner on Friday

The time word Pénteken makes the future meaning clear.

So even though the verb form is present, the whole sentence can refer to a future plan.