Jeg reagerer med det samme, når min chef sender en besked.

They are both in the present tense.

In Danish, the present tense is usually made by adding -r to the infinitive:

• at reagere → reagerer
• at sende → sender

So:

• Jeg reagerer = I react / I am reacting
• min chef sender = my boss sends / is sending

A useful point: Danish verbs do not change for person the way English verbs sometimes do. So you get:

• jeg reagerer
• du reagerer
• han/hun reagerer

The verb form stays the same.

Med det samme is an idiomatic expression meaning immediately, right away, or at once.

You should learn it as a whole phrase. If you translate the words one by one, it may seem odd, but in natural Danish it simply means without delay.

So:

• Jeg reagerer med det samme = I react immediately

A close synonym is straks, but med det samme is very common in everyday speech.

Because når means when in the sense of whenever or at the time that.

In this sentence, it describes a regular or expected situation:

• når min chef sender en besked = when / whenever my boss sends a message

Compare:

• når = when / whenever
• hvis = if
• da = usually when about a specific past event

So:

• Når min chef sender en besked, reagerer jeg med det samme = this happens whenever that situation occurs
• Hvis min chef sender en besked... = if that happens
• Da min chef sendte en besked... = when my boss sent a message on a particular past occasion

Because når min chef sender en besked is a subordinate clause.

In Danish main clauses, the finite verb usually comes in second position. But in subordinate clauses, that verb-second rule does not apply in the same way.

So here you get:

• når
  • min chef
    • sender
      • en besked

That is normal subordinate-clause order.

Compare:

• Main clause: Min chef sender en besked
• Subordinate clause: når min chef sender en besked

Then the main clause must still follow the Danish verb-second rule.

So the sentence becomes:

• Når min chef sender en besked, reagerer jeg med det samme.

Notice that you say reagerer jeg, not jeg reagerer, because the whole Når min chef sender en besked part takes the first position, and the finite verb of the main clause must come next.

That is a very important Danish word-order pattern.

Because Danish possessive words like min, din, hans, vores normally replace the article.

So you say:

• min chef = my boss
• min bil = my car
• vores hus = our house

Not:

• min chefen
• min bilen

The same thing happens in English: you say my boss, not my the boss.

Yes — in Danish, chef means boss, manager, or head of something.

This is a very useful false friend for English speakers.

In English:

• chef = professional cook

In Danish:

• chef = boss / manager
• kok = cook / chef in the English sense

So in this sentence, min chef definitely means my boss, not my cook.

Because Danish very often uses the present tense for future meaning when the context already makes the time clear.

Here, når already shows that we are talking about a future or repeated situation:

• Når min chef sender en besked, reagerer jeg med det samme.

This is completely natural Danish.

English does something similar in some clauses:

• When my boss sends a message, I react immediately.

You do not need a special future form here.

It depends on whether the negative is in the main clause or the subordinate clause.

If you negate the main clause:

• Jeg reagerer ikke med det samme, når min chef sender en besked.
• I do not react immediately when my boss sends a message.

Here ikke comes after the finite verb in the main clause.

If you negate the subordinate clause:

• Jeg reagerer med det samme, når min chef ikke sender en besked.

That sentence is grammatically fine, but it means something unusual: I react immediately when my boss does not send a message.

In subordinate clauses, ikke usually comes before the finite verb:

• når min chef ikke sender...

So this is an important contrast:

• Main clause: jeg reagerer ikke
• Subordinate clause: når min chef ikke sender

En besked means a message.

The noun besked is a common word for:

• a message
• a note
• information passed on to someone
• sometimes, in everyday modern use, a text/message depending on context

So this sentence could mean reacting when your boss sends:

• a text
• a chat message
• an email message
• some other kind of message

The exact type is not specified.

Also note:

• en besked = a message
• beskeden = the message

Good question. Danish often uses reagere på when you explicitly say what you are reacting to:

• Jeg reagerer på beskeden.
• I react to the message.

But in your sentence, the thing reacted to is not expressed as a direct på-phrase. Instead, the sentence focuses on when you react:

• Jeg reagerer med det samme, når min chef sender en besked.

So it means something like:

• I react immediately when my boss sends a message.

If you want to make the object more explicit, you could say:

• Jeg reagerer på beskeden med det samme.

That would mean I react to the message immediately.

Because når min chef sender en besked is a subordinate clause, and many Danish writers put a comma before such clauses.

So:

• Jeg reagerer med det samme, når min chef sender en besked.

That is very normal.

A useful note: in modern Danish, you may also see sentences without that comma, depending on the comma style being used. So both may occur in real life.

But for learners, it is very common and perfectly safe to write the comma before a subordinate clause like this.