Während der Kernarbeitszeit müssen alle im Büro sein.

Because während is a preposition here, and it normally takes the genitive case in standard German.

• die Kernarbeitszeit = nominative
• während der Kernarbeitszeit = genitive

So der is the genitive form of the feminine article die.

Since Kernarbeitszeit is a feminine noun, its genitive article is der.

Kernarbeitszeit is a compound noun:

• Kern = core
• Arbeitszeit = working time / working hours

So Kernarbeitszeit means something like core working hours: the period during the workday when everyone is expected to be present, even if they otherwise have flexible hours.

German often creates long nouns by combining smaller ones, and this is a very common example in workplace German.

It is capitalized because it is the first word of the sentence.

Normally, when während is used as a preposition, it is written lowercase:

• während der Pause
• während der Kernarbeitszeit

But at the beginning of a sentence, German capitalizes it, just like English does with sentence-initial words.

No. Während can also be a subordinating conjunction.

In your sentence, it is a preposition:

• Während der Kernarbeitszeit ...
• meaning during core working hours

As a conjunction, it would introduce a clause with a verb at the end:

• Während alle im Büro sind, arbeitet er konzentrierter.
• While everyone is in the office, he works more efficiently.

So the structure tells you which one it is:

• während + noun phrase -> preposition
• während + clause -> conjunction

Because German main clauses usually follow the verb-second rule.

The first element in the sentence is:

• Während der Kernarbeitszeit

Then the finite verb comes next:

• müssen

Then the subject:

• alle

So the structure is:

• Während der Kernarbeitszeit | müssen | alle | im Büro sein

This may feel unusual to English speakers, because English usually keeps the subject before the verb. But in German, the first position can be taken by many things, not just the subject.

You could also say:

• Alle müssen während der Kernarbeitszeit im Büro sein.

That version may feel more straightforward to English speakers.

Because alle here means everyone / all people, not everything.

• alle = all / everyone, referring to people in plural form
• alles = everything

In this sentence, the idea is that all employees / all people concerned must be in the office, so alle is the correct choice.

You can think of alle here as a pronoun standing for all persons.

Because im Büro describes location, not movement.

• im Büro = in dem Büro = in the office
• ins Büro = in das Büro = into the office

German uses different cases after in depending on meaning:

• dative for location: Where?
  • im Büro sein = to be in the office
• accusative for direction/movement: Where to?
  • ins Büro gehen = to go into the office

Since the sentence says people must be in the office, the dative form im Büro is required.

Because müssen is a modal verb, and with modal verbs German usually puts the main infinitive at the end.

Here:

• müssen = must
• sein = be

So:

• müssen ... sein = must be

This is very common in German:

• Ich muss arbeiten. = I must work.
• Wir müssen gehen. = We must go.
• Alle müssen im Büro sein. = Everyone must be in the office.

The finite modal verb takes the normal main-clause position, and the other verb goes to the end.

Yes, absolutely. That is also correct.

Both sentences mean the same thing:

• Während der Kernarbeitszeit müssen alle im Büro sein.
• Alle müssen während der Kernarbeitszeit im Büro sein.

The difference is mainly focus:

• Starting with Während der Kernarbeitszeit emphasizes the time period.
• Starting with Alle emphasizes who has the obligation.

German is flexible about what comes first, as long as the finite verb stays in second position in a main clause.

Often yes, but in everyday German müssen can sound very natural and common, not necessarily overly harsh.

In workplace rules like this, müssen expresses a real requirement or obligation:

• müssen = have to / must

Depending on context, English might translate it as either:

• must
• have to

So this sentence could naturally correspond to:

• Everyone must be in the office during core working hours.
• Everyone has to be in the office during core working hours.

Here alle is in the nominative, because it is the subject of the sentence.

You can see this from the structure:

• müssen alle ... sein = everyone must be ...

Even though the subject comes after the verb, it is still the subject, so it stays in the nominative case.

It is singular here.

• die Kernarbeitszeit = the core working time / the core working hours period

Even though English often uses the plural hours, German commonly uses the singular noun Arbeitszeit to refer to working time as a concept or scheduled period.

So während der Kernarbeitszeit is literally something like during the core working-time period.

Yes. In everyday spoken German, many people use dative after während, especially in informal speech.

So you may hear:

• während der Kernarbeitszeit
  • standard/preferred written form
• während der Kernarbeitszeit
  • in this specific sentence, the form looks the same anyway

That is because feminine singular der can already be genitive here, so you do not notice a difference.

With other nouns, the contrast is clearer:

• standard: während des Urlaubs
• colloquial: während dem Urlaub

For learners, it is best to remember:

• während + genitive is the safe standard rule.