Learn Hardcore Icelandic: Stelpan talar um tilfinningar sínar við vinkonu í stað þess að hugsa um þær ein. - The girl talks about her feelings with a female friend instead of thinking about them alone.

You could say talar um tilfinningu sína, but it would sound like she is talking about one specific feeling. The plural is much more natural here: tilfinningar sínar = her feelings in general.

Why is it sínar and not hennar? What’s the difference?

Both sínar and hennar can mean her, but they are used differently.

sínar is a reflexive possessive: it refers back to the subject of the clause (here, stelpan).
hennar is a non‑reflexive possessive: it refers to some other woman, not the subject.

So:

Stelpan talar um tilfinningar sínar…
= The girl talks about *her own feelings…* (the girl’s own feelings)
Stelpan talar um tilfinningar hennar…
= The girl talks about *her feelings…* (someone else’s feelings)

Here we want “her own feelings”, so sínar is correct.

Formally:

tilfinningar: feminine plural accusative
sínar: feminine plural accusative (agreeing with tilfinningar)

What is the case of tilfinningar sínar, and how does that affect the endings?

tilfinningar sínar is in the accusative plural, because it is the object of talar um.

Declension of tilfinning (feminine):

Nom pl: tilfinningar
Acc pl: tilfinningar
Dat pl: tilfinningum
Gen pl: tilfinninga

Declension of sinn in feminine plural:

Nom pl: sínar
Acc pl: sínar
Dat pl: sínum
Gen pl: sinna

So in the accusative plural, both happen to look like the nominative plural:

tilfinningar sínar (nom/acc fem pl) = her (own) feelings

The matching ‑ar / ‑ar endings show agreement in gender (fem) and number (pl).

Why is it við vinkonu and not með vinkonu? What’s the difference between við and með here?

Both við and með can be translated as with, but they are used differently.

tala við e-n = talk to someone / speak with someone (as an addressee)
tala með e-m is not idiomatic for speak with someone; með usually means with in the sense of together with / accompanied by.

So:

talar við vinkonu = talks to a (female) friend (the friend is the person she is talking to)
með vinkonu would sound more like with a friend (accompanied by a friend), and does not form the usual collocation for “talk to someone”.

Also, við takes the accusative, which is why vinkona becomes vinkonu:

Nom: vinkona (a female friend)
Acc: vinkonu → after við

What exactly does í stað þess að mean, and why do we need þess?

í stað þess að is a fixed expression meaning “instead of …‑ing”.

í stað = in place (of)
þess = genitive of það (it / that), literally “of that”
að
- verb = infinitive clause (to do / doing something)

So literally something like: in place of that (that) [she] thinks about them alone → instead of thinking about them alone.

In standard Icelandic you keep the þess:

í stað þess að hugsa… ✅ (correct, normal)
í stað að hugsa… ❌ (ungrammatical / non‑standard)

Learners can treat í stað þess að as an unbreakable chunk: “instead of …‑ing”.

What does þær refer to, and why is it þær and not something like þau or þeim?

þær is a third‑person feminine plural pronoun.

It refers back to tilfinningar (feelings):

tilfinningar: feminine plural
→ pronoun must also be feminine plural: þær

Case:

hugsa um e-ð takes the accusative
Feminine plural accusative of þær is þær (same form as nominative)

So:

hugsa um þær = think about them
where them = the feelings.

Other genders for comparison:

þau = neuter plural
þeir = masculine plural
þeim = dative plural (all genders), which would be wrong here because um requires accusative, not dative.

What does ein mean at the end, and what does it agree with?

ein here means “alone”, not “one” in a numerical sense.

The adjective einn/ein/eitt can mean:

one (the number), or
alone / by oneself (used predicatively)

In your sentence, ein is used in that “alone” meaning. It agrees with the subject, not the object:

subject: stelpan → feminine singular nominative
therefore: ein (feminine singular nominative)

So:

Stelpan er ein. = The girl is alone.
Stelpan hugsa um þær ein. = The girl thinks about them alone.

Even though ein comes after þær, it still describes stelpan (the one who is alone while thinking), not the feelings.

Other forms for “alone”:

masculine: einn – Hann er einn. (He is alone.)
feminine: ein – Hún er ein. (She is alone.)
neuter: eitt – Barnið er eitt. (The child is alone.)
plurals: einir / einar / ein depending on gender.

Why do we have um twice, in talar um and hugsa um? Is um always used with those verbs?

Yes, both tala um and hugsa um are standard verb + preposition combinations:

tala um e-ð = talk about something
hugsa um e-ð = think about something

In both, um introduces the thing being talked/thought about and takes the accusative.

You normally must include um with these meanings:

hugsa þær (without um) would not mean “think about them”; it would just look wrong here.
tala þær is also wrong; you need um (or a different preposition) to say what you are talking about.

So the two um’s are independent: one belongs to talar, the other to hugsa.

Can the word order be changed? For example, can I say Stelpan talar við vinkonu um tilfinningar sínar?

Yes, you can change the word order inside the clause without changing the meaning much. These are all grammatical:

Stelpan talar um tilfinningar sínar við vinkonu…
Stelpan talar við vinkonu um tilfinningar sínar…

Both mean essentially:

The girl talks to a (female) friend about her feelings…

The difference is very slight and mostly about emphasis / rhythm. Icelandic word order inside the clause is fairly flexible once the basic subject–verb position is respected.

You cannot, however, split up the fixed expression í stað þess að or move parts of it around. That chunk must stay together before the verb hugsa:

…í stað þess að hugsa um þær ein. ✅
…í stað að þess hugsa um þær ein. ❌

How would the sentence change if the subject were a boy instead of a girl?

If the subject is masculine (a boy), several forms would change to match the gender:

Stelpan (the girl, fem) → Strákurinn (the boy, masc)
sínar (fem pl) stays the same, because it agrees with tilfinningar, which are still feminine plural.
ein (fem sg) → einn (masc sg) to agree with the masculine subject.

A masculine version:

Strákurinn talar um tilfinningar sínar við vinkonu í stað þess að hugsa um þær einn.

Meaning: The boy talks about his feelings with a (female) friend instead of thinking about them alone.

Note:

þær stays feminine plural because it still refers to tilfinningar (fem pl).
Only the subject-related words (strákurinn, einn) change gender.

Is there a slightly more explicit or common way to say “by herself” than just ein?

Yes. A very common and clear way is:

ein og sér = literally alone and by herself
sjálf = herself (used for emphasis)

So you might hear:

…í stað þess að hugsa um þær ein og sér.
…í stað þess að hugsa um þær ein sjálf.

Both emphasise that she is on her own when thinking about them. Your original ein is already correct and idiomatic; these just add a bit more explicit emphasis.