Hún setur heilsuna sína í forgang þegar henni líður illa.

Heilsan means the health, and heilsuna is its accusative singular form. In Icelandic, it is very common for a noun to appear with the definite ending even when it is followed by a possessive like minn, þinn, sinn, etc.

So heilsuna sína is a normal Icelandic way to say her own health. It does not mean a strange English-style the her health; it is just how Icelandic structures this kind of phrase.

Because it is the direct object of the verb setur.

The verb setja means to put / set, and in this sentence she is putting her health into priority, so health is the thing being acted on. Direct objects are often in the accusative in Icelandic.

• nominative: heilsa
• accusative definite singular: heilsuna

So:

• Hún setur heilsuna sína í forgang.

Because sína is the reflexive possessive, and it refers back to the subject of the same clause.

Here, the subject is Hún (she), so sína means her own:

• Hún setur heilsuna sína í forgang = She prioritizes her own health

If you said heilsuna hennar, that would normally mean her health referring to some other female person, not the subject herself.

This is one of the most important uses of sinn in Icelandic.

Because sinn has to agree with the noun it belongs to in gender, number, and case.

Here it refers to heilsuna, which is:

• feminine
• singular
• accusative

So the correct form is:

• sína

Other forms of sinn include:

• sinn (masculine singular)
• sitt (neuter singular)
• sína (feminine singular accusative here, and also some plural forms depending on context)

That is the normal position for this kind of possessive in Icelandic, especially with sinn.

So:

• heilsuna sína = the usual, natural order

This is different from English, where possessives usually come before the noun. Icelandic often places them after the noun, especially when the noun has the definite ending.

Literally, it means something like puts ... into priority.

In natural English, that becomes:

• prioritizes
• puts first
• makes a priority

So setja eitthvað í forgang is an idiomatic expression meaning to treat something as a priority.

Here í is used with the accusative because there is a sense of movement or change of state: something is being put into priority.

The noun is forgangur (priority), and its accusative singular is:

• forgang

So:

• setja eitthvað í forgang

By contrast, í can also take the dative when it means location rather than movement/change, but that is not what is happening here.

Also, í forgang is a fixed and very common expression, so learners should memorize it as a phrase.

Because the verb líða in the sense to feel / to be doing takes the experiencer in the dative, not the nominative.

So the pronoun must be:

• nominative: hún
• dative: henni

That is why Icelandic says:

• henni líður illa

A very literal way to think of it is:

• to her, it feels bad

Even though English uses she, Icelandic uses the dative form henni here.

Because illa is an adverb, and the expression with líða uses an adverb:

• líða vel = feel well
• líða illa = feel bad / feel unwell

So you say:

• henni líður illa

not:

• henni líður ill

This is similar to how certain verbs in many languages naturally go with adverb-like expressions rather than adjectives.

It can mean:

• feels unwell
• feels bad
• is not feeling well

Depending on context, it can refer to:

• physical discomfort
• emotional distress
• a general sense of not feeling good

In this sentence, because it mentions putting health first, many learners will understand it as something like when she feels unwell or when she is not feeling well.

Þegar means when and introduces a subordinate clause of time:

• þegar henni líður illa = when she feels bad / when she feels unwell

So the sentence has:

• main clause: Hún setur heilsuna sína í forgang
• subordinate time clause: þegar henni líður illa

Yes. You can put the þegar-clause first:

• Þegar henni líður illa, setur hún heilsuna sína í forgang.

Notice that when the subordinate clause comes first, Icelandic keeps normal main-clause verb-second order:

• setur hún not
• hún setur

That word order is a very typical Icelandic pattern.

The infinitive is setja.

Setur is the 3rd person singular present:

• ég set
• þú setur
• hún/hann/það setur

So in this sentence:

• Hún setur ... = She puts / She sets

In this expression, English usually translates it more naturally as she prioritizes rather than she puts.