Ég skil alltaf töskuna mína eftir í aftursætinu, en hún setur sína í farþegasætið.

It means leave behind or simply leave in this sentence.

This is a very common Icelandic pattern: a verb + particle combination. The dictionary form is skilja eftir, but in a normal main clause the particle eftir is often separated from the finite verb:

• að skilja eftir = to leave behind
• Ég skil töskuna eftir = I leave the bag behind

So the split is normal and not something unusual.

It can be, yes. Ég skil by itself often means I understand.

But with eftir, the meaning changes:

• Ég skil. = I understand.
• Ég skil töskuna eftir. = I leave the bag behind.

So the context and the particle tell you which meaning is intended.

Because töskuna is the direct object, and it is:

• feminine
• singular
• definite
• accusative

The possessive mína has to match the noun in gender, number, and case.

So:

• taska mín = my bag
• töskuna mína = my bag (accusative definite form)

In Icelandic, possessives like mín usually come after the noun.

That is the normal Icelandic pattern.

English says:

• my bag

Icelandic usually says:

• taskan mín = my bag
• töskuna mína = my bag (in the accusative)

So for English speakers, the order can feel reversed at first, but it is completely standard in Icelandic.

Because sinn/sín/sitt is the reflexive possessive.

It is used when the owner is the subject of the same clause.

In:

• hún setur sína í farþegasætið

the subject is hún = she, and the bag is her own bag. That is why Icelandic uses sína.

If you used hennar, it would normally mean someone else’s bag, not the subject’s own bag.

So:

• hún setur sína ... = she puts her own one ...
• hún setur hennar ... would suggest someone else’s

It stands for the omitted noun bag.

The full version could be:

• ... en hún setur töskuna sína í farþegasætið.

But since bag is already understood from the first clause, Icelandic can leave the noun out and just use the possessive form:

• sína = hers / her own one

Its form is feminine singular accusative, because the understood noun is tösku/töskuna.

Because í can take either dative or accusative, depending on meaning.

• dative = location, being somewhere
• accusative = motion/direction into somewhere

So here:

• í aftursætinu = in the back seat
  This describes where the bag is left, so it is a location idea.

• í farþegasætið = into the passenger seat
  This describes movement/placement into that location, so it uses accusative.

This is a very important Icelandic pattern.

Both are definite singular neuter nouns, but in different cases.

• aftursæti = back seat

• aftursætinu = the back seat, dative singular

• farþegasæti = passenger seat
• farþegasætið = the passenger seat, accusative singular

The definite article in Icelandic is usually attached to the end of the noun, which is why these look like single words.

That is normal Icelandic word order.

In English, you usually say:

• I always leave my bag ...

In Icelandic, it is very natural to say:

• Ég skil alltaf töskuna mína eftir ...

So adverbs like alltaf often come after the finite verb. English speakers often notice this because English and Icelandic do not place adverbs in exactly the same way.

Yes, absolutely.

That would be the fuller version:

• Ég skil alltaf töskuna mína eftir í aftursætinu, en hún setur töskuna sína í farþegasætið.

Using just sína avoids repeating töskuna and sounds natural because the noun is already clear from context.

So both are possible, but the version with sína is more compact.

It has two main clauses joined by en = but:

1. Ég skil alltaf töskuna mína eftir í aftursætinu
2. en hún setur sína í farþegasætið

Very roughly:

• Ég = subject
• skil = finite verb
• alltaf = adverb
• töskuna mína = object
• eftir = particle
• í aftursætinu = prepositional phrase

Then:

• hún = subject
• setur = finite verb
• sína = object standing for her bag
• í farþegasætið = prepositional phrase

So even though the sentence looks long, it is built from two fairly straightforward clauses.