Learn Hardcore Swedish: Hon lyssnar på sina bästa låtar när hon studerar matematik. - She listens to her best songs when she studies mathematics.

hennes = her (non‑reflexive: someone else’s or not clearly tied to the subject)
sina / sin / sitt = her own / his own / their own (reflexive: belongs to the subject of the clause)

In this sentence, the subject is hon (“she”), and the songs belong to that same “she”. So Swedish uses the reflexive form:

Hon lyssnar på sina bästa låtar.
= She listens to her own best songs.

If you said:

Hon lyssnar på hennes bästa låtar.

this would usually mean: She listens to her (another woman’s) best songs. So sina shows that the owner and the subject are the same person.

Why do you say lyssna på and not just lyssna or lyssna till?

In modern Swedish, when you specify what you listen to, you almost always use:

lyssna på + object

Examples:

Hon lyssnar på musik. – She listens to music.
Vi lyssnar på radio. – We listen to the radio.
Jag lyssnar på dig. – I’m listening to you.

Lyssna till exists but is more formal/poetic or used in a few set phrases, and it’s less common in everyday speech.

You can use lyssna alone when you don’t mention the object:

Lyssna! – Listen!
Jag försökte lyssna. – I tried to listen.

But as soon as you name what you listen to (songs, music, someone), Swedish wants på.

What is the difference between lyssna and höra?

lyssna (på) = to listen (to) → intentional, active
höra = to hear → perception happens, whether you want it or not

So:

Hon lyssnar på sina bästa låtar.
She is actively listening to her best songs.
Hon hör musik genom väggen.
She hears music through the wall (she may not want to).

Using höra in the original sentence would be odd, because studying while passively hearing her best songs in the background is not the intended meaning.

Why is it bästa låtar and not something like mest bra låtar?

Bästa is the normal superlative form of bra (good):

bra – good
bättre – better
bäst / bästa – best

You do not say mest bra for “best”. You must use bäst/bästa.

About bäst vs bästa:

bäst is usually used on its own:
- Det är bäst. – That is best.
bästa is used before a noun:
- hennes bästa vän – her best friend
- sina bästa låtar – her best songs

So sina bästa låtar is exactly parallel to “her best songs” in English.

Why is it bästa låtar and not bästa låtarna or de bästa låtarna?

All three are possible, but they mean slightly different things:

sina bästa låtar
= her best songs (indefinite plural)
Neutral: just “her best songs” in general. That is what we have in the sentence.
sina bästa låtarna
This is usually not used; you would instead say:
sina bästa låtar
- context, or
de bästa låtarna (hon har) – the best songs (she has)

For a clearly definite meaning (“those particular best songs”), you’d say, for example:

Hon lyssnar på de bästa låtarna när hon studerar matematik.
= She listens to the best songs (the specific ones we have in mind) when she studies math.

In everyday speech, sina bästa låtar works very well to mean “her favourite/best songs” in a general, habitual sense.

Could the order be När hon studerar matematik lyssnar hon på sina bästa låtar? Is that still correct?

Yes, that is completely correct, just like in English:

Hon lyssnar på sina bästa låtar när hon studerar matematik.
När hon studerar matematik lyssnar hon på sina bästa låtar.

The difference is just emphasis and flow:

Starting with Hon: neutral, focus on what she does.
Starting with När hon studerar matematik: a bit more focus on the time/situation.

Notice that when the när‑clause comes first, Swedish keeps verb‑second (V2) word order in the main clause:

När hon studerar matematik lyssnar hon på sina bästa låtar.
After the initial När‑clause, the finite verb lyssnar comes before hon.

Why do you repeat hon in när hon studerar? Can you drop it like in some other languages?

No, you cannot drop hon. Swedish is not a “pro‑drop” language; subject pronouns are almost always required.

So you must say:

När hon studerar matematik lyssnar hon på sina bästa låtar.

You cannot say:

✗ När studerar matematik lyssnar på sina bästa låtar.

That would be ungrammatical.
Each clause (hon lyssnar…, hon studerar…) needs its own subject pronoun.

Why is studerar used, and what is the difference between studera, läsa, and plugga?

All three can mean something related to studying, but with different nuance:

studera – to study (formal/neutral, often about academic subjects)
- Hon studerar matematik. – She studies mathematics.
läsa – to read / to study (very common, can mean “read” or “take a course in”)
- Hon läser matematik. – She is taking/studying mathematics.
- Hon läser en bok. – She is reading a book.
plugga – to cram / study (informal, colloquial)
- Hon pluggar matte. – She’s cramming/studying math.

In a neutral example sentence in a textbook, studerar matematik is a typical choice, but in everyday speech you might hear läser matte or pluggar matte instead.

Why is it just matematik and not matematiken or something with an article?

Matematik is usually treated as a mass/abstract noun when you talk about the subject in general, just like “math” or “mathematics” in English:

Hon studerar matematik. – She studies mathematics.
Jag tycker om matematik. – I like mathematics.

You would not normally add an article here.

Matematiken (definite form) can be used in some contexts, for example when “the mathematics” has been specified or is treated as a concrete field:

Matematiken i det här kapitlet är svår.
– The mathematics in this chapter is difficult.

But in the sentence you gave, the simple, general matematik is the natural choice.

What is the singular form of låtar, and what gender is it?

The noun is:

singular indefinite: en låt – a song
singular definite: låten – the song
plural indefinite: låtar – songs
plural definite: låtarna – the songs

It is an en‑word (common gender).

So sina bästa låtar literally means “her own best songs” and uses the plural indefinite form låtar.