Hvis du er usikker på planen, skal du være opmærksom på datoen i kalenderen.

Hvis means if and introduces a condition.

So Hvis du er usikker på planen means If you are unsure about the plan.

In Danish, hvis starts a subordinate clause, which is why the first part of the sentence sets up the condition for the main clause that follows.

Because usikker normally takes the preposition på.

So in Danish, you say:

• at være usikker på noget = to be unsure about something

That means:

• usikker på planen = unsure about the plan

This is something you usually just have to learn together with the adjective: usikker på.

Planen is the definite form, meaning the plan.

Danish often adds the definite article to the end of the noun:

• en plan = a plan
• planen = the plan

So på planen means about the plan, not about a plan.

This is a very important Danish word-order rule.

In a main clause, the finite verb normally comes in the second position. When a subordinate clause comes first, as in:

• Hvis du er usikker på planen, ...

then the main clause still follows the verb-second rule, so you get:

• skal du være opmærksom på datoen i kalenderen

not:

• du skal være ...

This inversion is very common in Danish after a fronted element.

Compare:

• Du skal være opmærksom på datoen.
• Hvis du er usikker på planen, skal du være opmærksom på datoen.

Because the sentence has two clauses, and each clause needs its own subject.

• Hvis du er usikker på planen
• skal du være opmærksom på datoen i kalenderen

Even though it is the same person, Danish still repeats du just as English does:

• If you are unsure..., you should...

Here skal expresses something like should, must, or need to, depending on context.

In this sentence, it most naturally suggests advice or instruction:

• skal du være opmærksom på datoen
  = you should / need to pay attention to the date

So skal is not always a simple future tense word. It often carries an idea of obligation, necessity, or recommendation.

Yes, at være opmærksom på is a very common expression.

It means:

• to be aware of
• to pay attention to
• to be mindful of

So:

• vær opmærksom på det = be aware of that / pay attention to that

The preposition på belongs with the expression, so it is best learned as a whole chunk:

• opmærksom på noget

Because skal is a modal verb, and after modal verbs Danish uses the infinitive without at.

So:

• du skal være
• du kan være
• du må være

not:

• du skal at være

This works much like English:

• you must be
• you can be

not:

• you must to be

Because they mean the date and the calendar.

• en dato = a date
• datoen = the date
• en kalender = a calendar
• kalenderen = the calendar

So:

• datoen i kalenderen = the date in the calendar

The sentence is referring to a specific date and a specific calendar, which is why the definite forms are used.

In Danish, i kalenderen is the natural choice when you mean something written or entered in a calendar.

• i kalenderen = in the calendar

You may notice that English sometimes says on the calendar, but Danish usually prefers i kalenderen in this context.

So this is a case where Danish and English use different prepositions.

Not exactly. Opmærksom usually means attentive, aware, or paying attention.

In this sentence, være opmærksom på datoen means:

• pay attention to the date
• be aware of the date

If you wanted careful in the sense of avoiding danger or mistakes, Danish might use forsigtig instead, depending on the context.

So opmærksom is more about attention than caution.

Yes. You could say:

• Du skal være opmærksom på datoen i kalenderen, hvis du er usikker på planen.

That is also correct.

The difference is mainly one of emphasis and flow:

• Starting with Hvis du er usikker... puts the condition first.
• Starting with Du skal være opmærksom... puts the advice first.

Both are natural Danish.

The sentence has two parts:

1. Hvis du er usikker på planen
   = subordinate clause introduced by hvis

2. skal du være opmærksom på datoen i kalenderen
   = main clause

A useful breakdown is:

• Hvis = if
• du = you
• er = are
• usikker på planen = unsure about the plan
• skal = should / must / need to
• du = you
• være opmærksom på = be aware of / pay attention to
• datoen = the date
• i kalenderen = in the calendar

This is a very typical Danish pattern: subordinate clause first, then main clause with verb-second word order.