Learn Hardcore Croatian: Moja sestra igra košarku i stalno nosi loptu sa sobom, jer ne želi odustati od treninga. - My sister plays basketball and constantly carries the ball with her, because she doesn’t want to give up training.

Košarka is the base (nominative) form, but as the direct object of igrati, it must be in the accusative, which for a feminine noun like košarka is košarku.

Why is loptu used, and what case is it?

Loptu is the accusative singular of lopta (ball).

It’s the direct object of the verb nosi (she carries), answering “What does she carry?” → the ball.

Feminine nouns ending in -a usually form the accusative singular in -u:

lopta → loptu
knjiga → knjigu
sestra → sestru

What does stalno mean exactly, and how is it different from uvijek?

Stalno roughly means constantly / all the time / very often.

stalno – emphasizes continuous repetition or annoyingly frequent behavior
uvijek – literally always, in every case, at all times

In many contexts they overlap, but:

stalno nosi loptu sa sobom – she keeps carrying it all the time (habit, maybe slightly obsessive)
uvijek nosi loptu sa sobom – she always carries it, without exception

Why is it sa sobom and not s njom?

Sa sobom means “with herself” / “with her (own) self” and is reflexive: it refers back to the subject moja sestra.

sa sobom – with oneself (reflexive)
s njom – with her (some other “her”, not the subject)

In the sentence, the idea is: She carries the ball (together) with herself, i.e. she takes it wherever she goes. So reflexive sa sobom is natural.

What is the difference between s and sa in sa sobom?

Both s and sa mean with (also from in some contexts), but:

s is the basic form
sa is used:
- before some consonant clusters to make pronunciation easier
- often before words starting with s- or z-, like sa sobom, sa sestrom, sa zrnom

So you could technically say s sobom, but sa sobom is more natural and much easier to pronounce.

Why is there a comma before jer in ..., jer ne želi odustati od treninga?

In Croatian, jer (“because”) normally introduces a full subordinate clause, and you usually put a comma before it:

Ostaje kod kuće, jer je umorna. – She’s staying home, because she is tired.

So:
..., jer ne želi odustati od treninga.
“..., because she doesn’t want to give up training.”

The comma marks the start of the reason clause introduced by jer.

Why is it ne želi odustati (infinitive) and not ne želi da odustane?

Both are possible in Croatian, but they differ slightly in style and structure.

ne želi odustati
- verb željeti
  - infinitive
- very common, straightforward, somewhat more neutral/direct:
- literally: she doesn’t want to give up
ne želi da odustane
- verb željeti
  - clause with da
- also correct, slightly more “spoken” or colloquial in feel:
- literally: she doesn’t want that she give up

In your sentence, ne želi odustati is perfectly natural and probably the most common choice.

Why is the verb odustati used instead of odustajati?

Croatian verbs come in aspect pairs:

odustati – perfective (a completed action, “to give up (once)”)
odustajati – imperfective (ongoing, repeated process, “to be giving up / to give up repeatedly”)

Here we’re talking about the idea of giving up as a single act (she doesn’t want to give up her training), so the perfective odustati is the natural choice.

If you said:

ne želi odustajati od treninga,
it would sound like “she doesn’t want to keep (repeatedly) giving up on training,” which is a bit odd.

Why do we say odustati od treninga? What’s happening with od and treninga?

The verb odustati is almost always followed by the preposition od (to give up on / abandon something):

odustati od projekta – to give up on the project
odustati od plana – to abandon the plan

The preposition od always takes the genitive case, so trening must be in the genitive singular, which is treninga.

So:

odustati od treninga = to give up (on) training

Why is it moja sestra and not something like sestra moja?

Both moja sestra and sestra moja are grammatically correct, but:

moja sestra is the standard, neutral order (possessive adjective before the noun)
sestra moja sounds more:
- emotional / poetic, or
- used for emphasis or in vocative-like expressions: “Sestra moja!” – My sister!

In a normal descriptive sentence like this, Moja sestra igra košarku... is the natural word order.

Could we drop moja and just say Sestra igra košarku?

Yes, grammatically you can say Sestra igra košarku, but:

Without moja, it just means “(The) sister plays basketball”, which sounds incomplete or unclear in isolation.
Moja sestra makes it explicit that you mean my sister.

In normal conversation, if the context is clear (for example, the whole conversation is already about your sister), people might occasionally drop moja, but as a standalone sentence, Moja sestra is better.

Can the word order stalno nosi loptu sa sobom be changed?

Yes, Croatian word order is fairly flexible, and you can rearrange for emphasis:

Moja sestra stalno nosi loptu sa sobom. – neutral
Moja sestra loptu stalno nosi sa sobom. – emphasizes loptu (the ball)
Moja sestra nosi stalno loptu sa sobom. – slightly unusual, but possible, stressing stalno

The version in your sentence, igra košarku i stalno nosi loptu sa sobom, is the most natural and neutral.