Learn Hardcore Croatian: Mi treniramo u dvorani, a ostali treniraju u parku. - We train in the hall, and the others train in the park.

After the preposition u meaning in (location, static place), Croatian uses the locative case.
For feminine nouns like dvorana, the locative singular ending is usually -i:

u dvorani = in the hall / in the gym

So:

dvorana (dictionary form, subject)
u dvorani (in the hall – locative, after u for location)

Why is it u parku and not u park? What does the -u ending mean?

Park is a masculine noun:

base form (nominative singular): park

Again, u with a static location uses the locative case.
For many masculine nouns ending in a consonant, the locative singular ending is -u:

u parku = in the park

So:

park (dictionary form)
u parku (in the park – locative singular)

So both u dvorani and u parku are locative? How do I know when to use locative after u?

Yes, both are locative.

The preposition u can take different cases, but a simple rule:

u
- locative = in (location, being inside somewhere)
  - u dvorani – in the hall
  - u parku – in the park
u
- accusative = into (movement into somewhere)
  - Idemo u dvoranu. – We’re going into the hall.
  - Idem u park. – I’m going to the park.

In your sentence, the meaning is where the training happens (location), so locative is used.

Why is there no word for the in u dvorani and u parku? How do I know if it’s “in a hall” or “in the hall”?

Croatian has no articles (no words like a / an / the).

u dvorani can be translated as in a hall, in the hall, or in the gym, depending on context.
u parku can be in a park or in the park.

Context and previous mention usually tell the listener whether you mean a or the. The Croatian form itself does not change.

What does ostali mean exactly? Is it the same as drugi?

Ostali means the others / the rest.

Mi treniramo u dvorani, a ostali treniraju u parku.
= We train in the hall, and the others train in the park.

Details:

ostali is the masculine plural form (it agrees with an implied masculine or mixed group plural noun, like ljudi – people).
It has a nuance of the remaining ones / everyone else.

Drugi can also mean other, but:

ostali = the rest of the group you’re talking about
drugi = other / another, more general, can also mean second in some contexts

Here ostali is more natural because it contrasts “we” with “everyone else” in that group.

Could the sentence say Mi treniramo u dvorani, a oni treniraju u parku instead of ostali?

Yes, that’s also correct, but the meaning is slightly different:

Mi treniramo u dvorani, a oni treniraju u parku.
= We train in the hall, and they train in the park.
(just we vs they, two groups)
Mi treniramo u dvorani, a ostali treniraju u parku.
= We train in the hall, and the others train in the park.
(we are part of a larger group, and the rest train somewhere else)

Oni just refers to another group; ostali implies “all the remaining members of the same broader group.”

What is the role of a here? Why isn’t it i?

Both a and i can be translated as and, but they’re used differently:

i = simple addition, like and
- Mi treniramo u dvorani i treniramo u parku.
  (We train in the hall and train in the park – just adding information.)
a = and / while / whereas / but in a contrasting sense
It often contrasts two subjects or situations.

In your sentence:

Mi treniramo u dvorani, a ostali treniraju u parku.
= We train in the hall, while / whereas the others train in the park.

So a emphasizes the contrast between mi and ostali, and between u dvorani and u parku.

Could the word order be changed, like U dvorani mi treniramo, a u parku treniraju ostali? Does that change the meaning?

Yes, Croatian word order is flexible, and that version is grammatically correct:

U dvorani mi treniramo, a u parku treniraju ostali.

The basic meaning is the same, but emphasis shifts:

Mi treniramo u dvorani, a ostali treniraju u parku.
→ more neutral; slight focus on who does what.
U dvorani mi treniramo…
→ emphasizes the place first: In the hall, we train…
…a u parku treniraju ostali.
→ again focuses on the place first: and in the park, the others train.

Croatian uses word order partly for emphasis rather than strict grammar.

Does treniramo need a reflexive pronoun, like se treniramo?

In this sentence, no: Mi treniramo u dvorani is correct and natural.

trenirati can be:
- intransitive: Mi treniramo. – We train / practice.
- transitive: Mi treniramo nogomet. – We train football.

Se treniramo would sound wrong or at least very unusual here.
You might see forms like trenirati se in certain expressions, but for we train (work out / practice), just treniramo is used.

Is there any aspect difference in treniramo? Is it imperfective or perfective?

Trenirati is usually used as an imperfective verb in modern Croatian.

Mi treniramo u dvorani describes an ongoing, habitual, or repeated action:
We train / we are training in the hall.

There is a perfective partner istrenirati (to train up, to train fully), but:

You would not use istreniramo in this sentence, because you’re not describing a completed one-time result, but a regular or ongoing activity.

So treniramo here is imperfective present.