A naptáramban már benne van a pénteki határidő.

In Hungarian, possessive nouns often still take the definite article.

So:

• a naptáram = my calendar
• literally: the calendar-my

This is completely normal in Hungarian. English drops the, but Hungarian usually keeps a / az.

So A naptáramban... is not strange at all; it is the usual Hungarian way to say in my calendar.

It has three parts:

• naptár = calendar
• -am = my
• -ban = in

So:

• naptár → naptáram = my calendar
• naptáram
  • -ban → naptáramban = in my calendar

The m from -am and the b of -ban come together naturally, so you get naptáramban.

Már means already.

It shows that this is not new anymore: the Friday deadline has been entered and is now in the calendar.

Compare:

• A naptáramban benne van a pénteki határidő. = The Friday deadline is in my calendar.
• A naptáramban már benne van a pénteki határidő. = The Friday deadline is already in my calendar.

So már adds the idea of it has already happened / it is already the case.

Benne van is a very common Hungarian expression meaning is in it / is inside / is included.

Here:

• benne = in it, inside
• van = is

Together, benne van often means not just physical location, but also is included, is entered, or is there.

In this sentence, it sounds natural because a deadline being in a calendar is like saying it has been entered or included there.

Using just van is sometimes possible in other structures, but benne van is a very idiomatic way to say this.

Yes, from an English point of view it can feel repetitive, but in Hungarian this is natural.

• A naptáramban sets the location/topic: in my calendar
• benne van then says that something is in there / is included there

So Hungarian can use both:

• the full noun phrase: a naptáramban
• and the matching adverb-like form: benne

This kind of doubling is common and idiomatic, especially when the location is brought to the front for emphasis or as the topic of the sentence.

Péntek means Friday.

The ending -i turns it into an adjective:

• péntek = Friday
• pénteki = Friday, Friday-related, for Friday, on Friday

So:

• a pénteki határidő = the Friday deadline / the deadline for Friday

This -i ending is very common in Hungarian for turning nouns, places, or times into adjectives:

• budapesti = from Budapest / Budapest-related
• nyári = summer, summer-related
• hétfői = Monday, Monday-related

Because határidő is the subject here, not the direct object.

Hungarian uses -t for the direct object:

• határidő = deadline
• határidőt = deadline as an object

But in this sentence, the deadline is the thing that is in the calendar, so it is the subject:

• a pénteki határidő = the Friday deadline

That is why it stays in the basic form, without -t.

Hungarian word order is strongly connected to topic and focus, not just grammar.

In this sentence:

• A naptáramban = the topic or setting: as for my calendar / in my calendar
• már = already
• benne van = is in there / is included
• a pénteki határidő = the thing being talked about

So the sentence is organized roughly like this:

As for my calendar, the Friday deadline is already in there.

This is why the order may feel unusual to an English speaker. Hungarian often puts the known setting first.

Yes, that is also possible.

It would mean essentially the same thing:

• A pénteki határidő már a naptáramban van.
• A naptáramban már benne van a pénteki határidő.

The difference is mostly in emphasis and style.

The original sentence starts with A naptáramban, so it puts the calendar first as the topic. It sounds a bit like:

• In my calendar, the Friday deadline is already there.

Your alternative starts with A pénteki határidő, so it puts the deadline first:

• The Friday deadline is already in my calendar.

Both are good; they just organize the information differently.

Yes, effectively it does.

Benne means in it / inside it, and in this sentence the it is understood from a naptáramban.

So even though naptáramban is already stated, benne points back to that location.

This is one reason the sentence feels very natural in Hungarian: the location is named, and then the sentence continues with a matching form meaning in there.

It can definitely suggest more than physical location.

With calendars, lists, schedules, and documents, benne van often means:

• is in it
• is included
• has been entered
• is on it

So this sentence can naturally imply:

• the deadline is written in the calendar
• it has already been added
• it is already noted down

That is why the expression sounds very natural in this context.