Learn Hardcore Norwegian: Da bekymringen hennes var størst, kom en smilende sykepleier og forklarte hva samtykke betyr for pårørende. - When her worry was greatest, a smiling nurse came and explained what consent means for relatives.

Da is used for a specific time in the past (a particular moment/occasion): “when (that time)”.
Når is typically used for habitual/repeated events or general time (“whenever/when in general”), and often for present/future contexts.
So Da bekymringen hennes var størst points to one specific moment when her worry peaked.

Why is there a comma after størst?

Because the sentence begins with a subordinate clause (Da ...). Norwegian normally places a comma between an initial subordinate clause and the main clause:

Da bekymringen hennes var størst, (subordinate clause)
kom en smilende sykepleier ... (main clause)

Why is the word order kom en smilende sykepleier and not en smilende sykepleier kom?

Norwegian is a V2 language in main clauses: the finite verb must be in the second position.
Since the sentence starts with the subordinate clause (Da ...), the main clause begins after the comma, and then the verb comes first:

..., kom (verb) en smilende sykepleier (subject)

If you started directly with the subject, you’d get normal subject-first order:

En smilende sykepleier kom og forklarte ...

What does bekymringen hennes mean grammatically, and why is bekymringen in the definite form?

bekymring = “worry/concern”.
bekymringen is the definite singular form (“the worry”). With hennes (“her”), Norwegian commonly uses double definiteness: definite noun + possessive:

bekymringen hennes = “her worry” (literally “the worry hers”)

You can also place the possessive before the noun, but then the noun is usually indefinite:

hennes bekymring = “her worry” (slightly more formal/less common in everyday style)

Why is it var størst and not something like var den største?

størst is the superlative of stor (“big/great”), used predicatively after var (“was”):

bekymringen var størst = “the worry was greatest / at its greatest”

You typically don’t use den største here because that form is common when the superlative is used attributively with a noun phrase like “the greatest worry (of several)”:

den største bekymringen = “the greatest worry”

Here the meaning is “her worry was at its peak,” so var størst fits well.

What is smilende—an adjective or a verb form?

smilende is the present participle of å smile (“to smile”), used like an adjective meaning “smiling”:

en smilende sykepleier = “a smiling nurse”

Norwegian often uses present participles this way to describe someone’s appearance or manner.

Why does it say en smilende sykepleier—what gender is sykepleier?

sykepleier (“nurse”) is a common-gender noun, so the indefinite article is en:

en sykepleier (a nurse)

If it were neuter, it would take et.

Why are there two verbs: kom ... og forklarte?

This is a common Norwegian pattern: two coordinated past actions with og (“and”):

kom = “came”
forklarte = “explained”

It implies sequence or connected actions: the nurse arrived and then explained.

Why is it forklarte hva samtykke betyr (present tense betyr) instead of betydde?

Norwegian often uses the present tense in an embedded clause to express a general truth/definition that is still valid:

forklarte hva samtykke betyr = “explained what consent means”

Using betydde would sound more like “meant at that time (in that context)”, which is usually not what you want for definitions.

Why is the order hva samtykke betyr and not hva betyr samtykke?

Because hva samtykke betyr is an embedded question (an indirect question) after forklarte. In embedded questions, Norwegian uses statement word order (subject before verb):

direct question: Hva betyr samtykke? (“What does consent mean?”)
embedded: ... forklarte hva samtykke betyr.

What does samtykke look like as a noun (gender/definite form), and is it countable?

samtykke is usually neuter:

et samtykke (a consent)
samtykket (the consent)

In many contexts it’s used more like an uncountable concept (“consent” in general), but it can also be countable when referring to a specific consent/authorization.

What does pårørende mean here, and is it singular or plural?

pårørende refers to next of kin / relatives / close family members (often in a medical context). It’s frequently used as a collective/general term.
Form-wise, pårørende can function as plural (“relatives/next of kin”), and it’s also commonly used in a general sense without specifying number:

for pårørende = “for next of kin / for relatives”

Why is it for pårørende—what does for mean in this context?

Here for means “for / with regard to / as it applies to”:

hva samtykke betyr for pårørende = “what consent means for next of kin” (i.e., in their situation/role)