Vær opmærksom på datoen i kalenderen, hvis du ikke vil komme for sent til mødet.

Vær is the imperative form of at være.

So Vær opmærksom ... means Be attentive / Pay attention ...

Danish often uses the imperative to give instructions, advice, or warnings:

• Vær stille. = Be quiet.
• Kom her. = Come here.
• Husk nøglerne. = Remember the keys.

So this sentence is giving advice directly to you.

Here opmærksom is a predicate adjective after vær.

In Danish, adjectives used after være/blive/se ud usually stay in the basic form when they describe a person in the common gender singular or an implied you:

• Du er træt.
• Han er klar.
• Vær opmærksom.

You would use -t in other situations, for example with a neuter singular noun:

• Et barn er opmærksomt.

But in Vær opmærksom, the adjective describes the understood du, so opmærksom is correct.

Yes. Opmærksom normally goes with the preposition på when it means aware of / attentive to / paying attention to.

So:

• være opmærksom på noget = to be aware of something / pay attention to something

Examples:

• Vær opmærksom på trafikken.
• Hun er opmærksom på problemet.

This is one of those combinations that is best learned as a set phrase: opmærksom på

Because Danish usually expresses the by adding a definite ending to the noun.

So:

• en dato = a date

• datoen = the date

• en kalender = a calendar

• kalenderen = the calendar

• et møde = a meeting
• mødet = the meeting

The endings depend on the noun:

• common gender nouns often take -en
• neuter nouns often take -et

Here the speaker means a specific date, a specific calendar, and a specific meeting, so the definite forms are used.

I kalenderen is the most natural choice here because the date is thought of as something inside the calendar system or written as an entry in it.

So:

• i kalenderen = in the calendar

You may sometimes hear på kalenderen in certain contexts, especially when talking about something shown on a digital calendar or scheduled on someone's calendar. But in this sentence, i kalenderen is very natural and standard.

For a learner, it is safest to remember:

• en dato i kalenderen

Hvis means if and introduces a conditional clause.

So:

• hvis du ikke vil komme for sent til mødet
  = if you do not want to arrive late for the meeting

It sets up the condition for the advice in the main clause:

• Be attentive to the date in the calendar, if you do not want to be late ...

Very common uses:

• Hvis det regner, bliver vi hjemme.
• Hvis du har tid, så ring.

This is normal Danish word order in a subordinate clause.

After a subordinating word like hvis, Danish usually has: subject + sentence adverb + finite verb

So:

• hvis du ikke vil ...
• subject = du
• sentence adverb = ikke
• finite verb = vil

Compare that with a main clause, where the finite verb usually comes earlier:

• Du vil ikke komme for sent.

So the position of ikke helps show that this is a subordinate clause.

Here vil is best understood as want to, not simple future.

So:

• hvis du ikke vil komme for sent
  = if you do not want to arrive late

In Danish, ville/vil can express both:

1. future
2. wish / intention / willingness

In this sentence, the meaning is clearly about preference or intention:

• you do not want to be late

If it were pure future, the sentence would sound strange in context.

For sent means too late / late in the expression komme for sent.

This is a fixed and very common Danish expression:

• at komme for sent = to arrive late

Examples:

• Jeg kom for sent til skole.
• Hun kommer altid for sent.

Even though for often means too, in this expression you should learn the whole phrase as a unit: komme for sent

The preposition til is used with komme when talking about arriving to / for an event, place, or appointment.

So:

• komme til mødet = come to the meeting
• komme for sent til mødet = arrive late for the meeting

Also, mødet is definite because it refers to a specific meeting already known in the situation.

Compare:

• et møde = a meeting
• mødet = the meeting

Because Danish spelling rules normally use a comma before a subordinate clause introduced by a word like hvis, at, fordi, da, etc.

So:

• Vær opmærksom på datoen i kalenderen, hvis du ikke vil komme for sent til mødet.

That comma helps separate the main clause from the conditional clause.

In other words:

• main clause: Vær opmærksom på datoen i kalenderen
• subordinate clause: hvis du ikke vil komme for sent til mødet

Yes, but it would not mean exactly the same thing.

• Vær opmærksom på datoen means pay attention to / be mindful of the date
• Se på datoen means look at the date

So Vær opmærksom på is broader. It is not just about physically looking; it is about remembering and taking the date seriously.

That makes Vær opmærksom på datoen i kalenderen sound more like advice or a warning, which fits this sentence well.