Det kliar i hennes ögon när hon sitter ute för länge.

Because det kliar is a very common Swedish pattern meaning it itches. The det here is a formal/dummy subject, just like English it in it hurts or it rains. It does not refer to a specific thing.

So:

• Det kliar = It itches
• Det svider = It stings
• Det gör ont = It hurts

Swedish often needs this kind of subject in sentences like this.

Kliar is the present tense of klia, which is related to itching.

A useful distinction is:

• Det kliar = It itches
• Jag kliar mig = I scratch myself
• Jag kliar henne på ryggen = I scratch her back

So in your sentence, kliar does not mean that someone is scratching. It means that the eyes feel itchy.

Both kinds of structure are possible in Swedish, but they are used a little differently.

• Det kliar i hennes ögon = Her eyes are itchy / It itches in her eyes
• Hennes ögon kliar = also possible, but a bit more direct and less common in this exact everyday phrasing

Swedish very often uses det + verb + i/kroppsdel for bodily sensations:

• Det gör ont i ryggen = My back hurts
• Det svider i halsen = My throat stings
• Det kliar i ögonen = My eyes itch

So i hennes ögon is a very natural body-sensation construction.

Because after i in this kind of expression, Swedish often uses the noun without the definite article.

So:

• i ögonen can occur in some contexts
• but det kliar i ögonen is a very common fixed expression
• det kliar i hennes ögon is also natural

When a possessive like hennes is used, Swedish does not also add the definite ending:

• hennes ögon = her eyes
• not hennes ögonen

That is the normal Swedish rule: a possessive already makes the noun definite in meaning.

Because hennes means her, while sin/sitt/sina is a special reflexive possessive meaning his/her/their own and usually refers back to the subject of the same clause.

Here the relevant clause is Det kliar i hennes ögon. The subject there is det, not hon. Since hon is not the grammatical subject of that clause, sina would not fit naturally here.

So:

• Hon tvättar sina händer. = She washes her own hands.
• Det kliar i hennes ögon. = Her eyes itch.

Even though hon appears later in the sentence, it is inside another clause: när hon sitter ute för länge.

Here när means when.

It introduces a subordinate clause:

• när hon sitter ute för länge = when she sits outside too long

So the full sentence means that the itching happens when that situation occurs.

In other contexts, när can also refer to time in questions:

• När kommer du? = When are you coming?

Because när starts a subordinate clause, and Swedish word order changes in subordinate clauses.

Main clause:

• Det kliar i hennes ögon

Subordinate clause:

• när hon sitter ute för länge

In a subordinate clause, Swedish normally puts things in the order subject + verb, and if there is a sentence adverb like inte, it usually comes before the verb’s main complement position:

• Jag vet att hon inte kommer.
• när hon inte sitter ute

So the sentence follows normal Swedish grammar.

Literally, yes: sitter ute means sits outside.

But in Swedish, verbs like sitta, stå, and ligga are often used more naturally than in English to describe someone’s position or state. Depending on context, sitter ute can mean:

• is sitting outside
• is outside
• stays outside

In this sentence, it probably means she is spending time outdoors, likely seated, for too long.

A similar Swedish habit is:

• Boken ligger på bordet. = The book is on the table
  (literally lies on the table)

Both are possible, but they are not exactly the same.

• är ute = is outside / is outdoors
• sitter ute = is sitting outside

So sitter ute is more specific. It paints a clearer picture: she is outside, probably sitting there for a while. That fits well with the idea of irritation in her eyes from being outdoors too long.

If you said Det kliar i hennes ögon när hon är ute för länge, that would also be understandable and natural, but slightly less specific.

Here för länge means for too long or too long.

Important distinction:

• länge = for a long time
• för länge = too long

Examples:

• Hon satt ute länge. = She sat outside for a long time.
• Hon satt ute för länge. = She sat outside too long.

So för adds the idea that the duration is excessive.

Yes, but only in a different context.

Swedish för länge sedan means a long time ago, and för sent means too late. On its own, för länge usually means too long, not too late.

So in this sentence:

• för länge = too long

There is no ambiguity here because it clearly refers to duration.

Yes. Ute is an adverb meaning out, outside, or outdoors.

It describes location/state, not a noun. That is why there is no article or noun after it.

Compare:

• Hon sitter ute. = She is sitting outside.
• Hon sitter i trädgården. = She is sitting in the garden.

So ute works on its own as a location adverb.

Yes. Here are a few natural alternatives, with slight differences in tone or focus:

• Hennes ögon kliar när hon sitter ute för länge.
  More direct: Her eyes itch when she sits outside too long.

• Det börjar klia i hennes ögon när hon sitter ute för länge.
  Her eyes start to itch when she sits outside too long.

• Hon får kliande ögon när hon sitter ute för länge.
  She gets itchy eyes when she sits outside too long.

The original version is very idiomatic and common because Swedish often uses det + sensation verb + i + body part.

It has two parts:

1. Main clause:
   Det kliar i hennes ögon
   = Her eyes itch / It itches in her eyes

2. Subordinate clause introduced by när:
   när hon sitter ute för länge
   = when she sits outside too long

So the overall structure is:

[main clause] + [när-clause]

That is a very common Swedish sentence pattern.