Jeg reagerer roligt, når nogen afbryder mig.

Because the verb at reagere belongs to a very common Danish pattern where the present tense ends in -r.

• at reagere = to react
• jeg reagerer = I react / I am reacting

This is normal in Danish:

• jeg taler = I speak
• jeg bor = I live
• jeg arbejder = I work

So reagerer is simply the present tense form.

Roligt means calmly here.

It comes from the adjective rolig, which means calm. In this sentence, it is being used like an adverb, describing how the person reacts.

• rolig = calm
• roligt = calmly / in a calm way

Danish often uses the -t form of an adjective when it functions adverbially:

• han taler højt = he speaks loudly
• hun arbejder hurtigt = she works quickly
• jeg reagerer roligt = I react calmly

So the -t is not random; it is the normal form used here.

Når is used for something that happens whenever or when in a general or repeated sense.

In this sentence:

• når nogen afbryder mig = when someone interrupts me / whenever someone interrupts me

This suggests a general situation, not one specific past event.

By contrast, da is often used for:

• a specific moment in the past
• or sometimes when/since depending on context

Compare:

• Jeg reagerer roligt, når nogen afbryder mig. = I react calmly when someone interrupts me.
• Jeg reagerede roligt, da han afbrød mig. = I reacted calmly when he interrupted me.

So når is the right choice for a general repeated situation.

Here, nogen means someone.

So:

• nogen afbryder mig = someone interrupts me

Useful contrasts:

• nogen = someone / anyone
• ingen = no one / nobody
• nogle = some

Examples:

• Er der nogen hjemme? = Is anyone home?
• Ingen kom. = No one came.
• Nogle mennesker er meget højrøstede. = Some people are very loud.

In your sentence, nogen is singular in meaning: someone.

It is not actually at the very end, because mig comes after it, but it is in the normal position for a verb in a subordinate clause.

Danish word order changes depending on whether the clause is:

• a main clause
• or a subordinate clause

Main clause:

• Jeg reagerer roligt.

Subordinate clause introduced by når:

• når nogen afbryder mig

In Danish subordinate clauses, the subject usually comes before the finite verb:

• når nogen afbryder mig
• fordi jeg er træt
• hvis han kommer

That is standard Danish syntax.

The base form is at afbryde, which means to interrupt.

Breakdown:

• af- = a prefix
• bryde = break

Historically/literally, it is something like break off, but in modern usage it commonly means:

• to interrupt a person
• to cut something off
• to discontinue something

In this sentence:

• nogen afbryder mig = someone interrupts me

Other examples:

• Undskyld, at jeg afbryder. = Sorry for interrupting.
• Hun afbrød mødet. = She interrupted / called off the meeting.

Because mig is the object form of jeg.

Danish pronouns change form depending on their role:

• jeg = I
• mig = me

In this sentence:

• jeg is the subject of reagerer
• mig is the object of afbryder

So:

• Jeg reagerer roligt = I react calmly
• når nogen afbryder mig = when someone interrupts me

Just like in English:

• I react calmly when someone interrupts me.

Usually, no—not if you mean someone interrupts me.

Nogen is the natural choice for someone/anyone:

• når nogen afbryder mig

Nogle usually means some in the plural:

• nogle mennesker = some people

So nogle afbryder mig would sound incomplete or odd in most contexts unless something is left understood. For a simple sentence meaning when someone interrupts me, stick with nogen.

It can mean either, depending on context.

The Danish present tense often covers both:

• I react calmly
• I am reacting calmly

So:

• Jeg reagerer roligt can be a general statement about your behavior
• or, in the right context, it can describe what is happening now

In this full sentence, the most natural meaning is usually the general/habitual one:

• I react calmly when someone interrupts me.

Yes. If you start with the når-clause, Danish uses normal V2 word order in the main clause.

Original:

• Jeg reagerer roligt, når nogen afbryder mig.

Also possible:

• Når nogen afbryder mig, reagerer jeg roligt.

Notice what happens:

• the subordinate clause comes first
• then the verb reagerer comes before the subject jeg

This is a very important Danish pattern:

• Når jeg har tid, læser jeg.
• Hvis det regner, bliver vi hjemme.

So both versions are correct, but the word order changes.

Yes—both may be a little tricky for English speakers.

Roligt

• The spelling may make you expect every sound clearly, but Danish pronunciation is often more reduced than the spelling suggests.
• The g is not pronounced like a hard English g here.
• The word may sound softer and more compressed than learners expect.

Afbryder

• The y sound in Danish is difficult for many English speakers. It is not the same as English oo or ee.
• The word also has Danish r sounds that may be hard at first.

A good strategy is to learn these as whole sound patterns rather than trying to pronounce each letter one by one. Listening and repeating native audio is especially useful for Danish.

Yes, it is natural and correct Danish.

It sounds like a clear, neutral statement about someone’s typical reaction:

• Jeg reagerer roligt, når nogen afbryder mig.

A native speaker might also say similar things depending on nuance:

• Jeg tager det roligt, når nogen afbryder mig. = I stay calm when someone interrupts me.
• Jeg bliver ikke vred, når nogen afbryder mig. = I do not get angry when someone interrupts me.

But your original sentence is perfectly natural and idiomatic.