Learn Hardcore Norwegian: Sykepleieren reagerer øyeblikkelig når pasienten sier at smerten blir verre. - The nurse reacts immediately when the patient says that the pain is getting worse.

In this sentence, we are talking about a specific nurse, a specific patient, and that patient’s specific pain, so the definite form is used, just like in English:
The nurse reacts immediately when the patient says that the pain is getting worse.

Could we say en sykepleier reagerer instead of sykepleieren reagerer?

Yes, but the meaning changes:

Sykepleieren reagerer … = The nurse reacts … (a specific nurse)
En sykepleier reagerer … = A nurse reacts … (any nurse; this is more general or hypothetical)

So en sykepleier would sound more like you are describing what typically happens or giving an example, not reporting a concrete situation with a known nurse.

What is the function of når here, and how is it different from da?

Når introduces a time clause and here corresponds to English when:

når pasienten sier at smerten blir verre
= when the patient says that the pain is getting worse

In general:

når is used for:
- present and future events:
  Når jeg kommer hjem, spiser jeg. = When I get home, I eat.
- repeated or general events in the past:
  Når jeg var liten, lekte jeg mye ute. = When(ever) I was little, I played outside a lot.
da is used for:
- one specific event in the past:
  Da jeg kom hjem, spiste jeg. = When I came home (that time), I ate.

In your sentence, this is a general situation (whenever this happens), so når is the correct choice.

Why is reagerer in the present tense, not a future form?

Norwegian uses the present tense very often where English might use a more future-looking form:

Sykepleieren reagerer øyeblikkelig når …
Literally: The nurse reacts immediately when …

This describes what typically happens or what always happens in that situation. It is a general rule or habit, not a one-time future event.

Norwegian does have future expressions (skal, kommer til å), but in når-clauses and general statements like this, the plain present tense is normal and natural.

In English we say reacts to something. Why is there no på in reagerer øyeblikkelig?

Norwegian å reagere can be:

intransitive (no preposition):
Hun reagerer raskt. = She reacts quickly.
Sykepleieren reagerer øyeblikkelig. = The nurse reacts immediately.
or with på when you react to something specific:
Hun reagerer på kritikken. = She reacts to the criticism.
Sykepleieren reagerer på smertene. = The nurse reacts to the pain.

In your sentence, the nurse is generally reacting / taking action when the situation occurs, so reagerer is used without på. The når-clause tells us under which condition she reacts, not what direct object she reacts to.

What does øyeblikkelig mean exactly, and are there simpler synonyms?

Øyeblikkelig is an adverb meaning immediately, at once, without delay.

Common, simpler synonyms:

med en gang = right away
straks = immediately
umiddelbart = immediately, without delay

So you could also say:

Sykepleieren reagerer med en gang …
Sykepleieren reagerer straks …
Sykepleieren reagerer umiddelbart …

Where can øyeblikkelig go in the sentence? Is the word order flexible?

In a simple main clause, øyeblikkelig normally comes after the verb:

Sykepleieren reagerer øyeblikkelig når … ✅ (neutral, natural)

You can also move it to the front for emphasis:

Øyeblikkelig reagerer sykepleieren når … ✅ (more dramatic / literary)

But you would not normally put it between the subject and the verb:

✗ Sykepleieren øyeblikkelig reagerer … (sounds wrong/unnatural)

So the safe pattern to remember: Subject – Verb – Adverb
Sykepleieren – reagerer – øyeblikkelig

Why do we say pasienten sier at smerten blir verre and not just pasienten sier smerten blir verre?

In Norwegian, the conjunction at usually has to be used to introduce a that-clause (a content clause):

Han sier at smerten blir verre.
= He says that the pain is getting worse.

In English, that is optional here; you can say:

He says that the pain is getting worse.
He says the pain is getting worse.

In Norwegian, dropping at is much less common and more restricted. In standard written Norwegian, you should include at in sentences like this.

In at smerten blir verre, why is the word order smerten blir, not blir smerten?

Norwegian has two main word-order patterns:

Main clauses (independent sentences) follow the V2 rule: the verb is in second position:
- Smerten blir verre. = The pain is getting worse.
- Da blir smerten verre. = Then the pain gets worse.
Subordinate clauses (introduced by at, når, fordi, som, etc.) use regular Subject–Verb order:
- at smerten blir verre (that the pain is getting worse)
- når smerten blir verre (when the pain gets worse)

Your phrase at smerten blir verre is a subordinate clause, so the correct order is Subject – Verb: smerten blir.

What is the difference between blir verre and er verre?

blir verre = is becoming / is getting worse, describes a change or worsening process
- smerten blir verre = the pain is getting worse (it’s increasing)
er verre = is worse, describes a state
- smerten er verre i dag = the pain is worse today (compared to before)

In your sentence, the patient is telling the nurse that the pain is worsening, so blir verre is more precise.

Could we move the når-clause to the beginning of the sentence, and does the word order change?

Yes, you can move it to the front:

Når pasienten sier at smerten blir verre, reagerer sykepleieren øyeblikkelig.

In Norwegian main clauses, the finite verb must be in second position (V2 rule). When you move the når-clause to the front, that whole clause counts as position 1, so the next element must be the verb reagerer, and the subject sykepleieren comes after it:

Når pasienten sier at smerten blir verre, (position 1)
reagerer (position 2, the verb)
sykepleieren (position 3, the subject)
øyeblikkelig.

How is smerten formed from smerte, and what are its gender and plural forms?

The base noun is smerte (pain). It is usually treated as masculine in Bokmål:

en smerte = a pain
smerten = the pain

Plural:

smerter = pains
smertene = the pains

So smerten is the pain in the singular definite form, just like sykepleieren (the nurse) and pasienten (the patient).

How do you pronounce sykepleieren, øyeblikkelig, and verre?

Approximate pronunciations (standard Eastern Norwegian):

sykepleieren ≈ SY-keh-plei-ren
- sy like English see but with rounded lips
- stress on SY: SY-ke-plei-ren
øyeblikkelig ≈ ØY-eh-BLIK-ke-lig
- øy like the oy in boy, but with more rounded lips
- stress on ØY: ØY-e-blik-ke-lig
verre ≈ VEH-re
- short e like in bed
- double r is rolled or tapped
- stress on VEH: VEH-re

These are only rough guides, but they should help you get reasonably close.