Learn Hardcore Croatian: Glazbenik često vježba u tihoj sobi. - The musician often practices in a quiet room.

So the same English word musician can be either glazbenik (m.) or glazbenica (f.) in Croatian, depending on the person’s gender. Context tells you which one is meant if it’s not specified.

Why is there no word for a/the in Glazbenik često vježba u tihoj sobi?

Croatian does not use articles like a/an or the.

Glazbenik can mean a musician or the musician, depending on context.
u tihoj sobi can be understood as in a quiet room or in the quiet room.

Definiteness is usually clear from context, not from a specific word as in English.

Where can the adverb često go in the sentence? Is Glazbenik često vježba… the only correct word order?

Često means often and is usually placed before the verb it modifies:

Glazbenik često vježba u tihoj sobi.

Other positions are possible, but they change emphasis or can sound less neutral:

Često glazbenik vježba u tihoj sobi. – Emphasis on often (as the first word).
Glazbenik vježba često u tihoj sobi. – Possible, sounds a bit heavier; still fine in speech for emphasis.

The most natural neutral position is right before the verb: često vježba.

What is the difference between vježbati and svirati? Can I say Glazbenik često svira u tihoj sobi?

vježbati = to practice (a skill/activity in general):
- vježbati klavir – to practice the piano
- vježbati govor – to practice a speech
svirati = to play (an instrument):
- svirati gitaru – to play the guitar
- svirati klavir – to play the piano

In your sentence:

Glazbenik često vježba u tihoj sobi. – Focus on practice as an activity (rehearses).
Glazbenik često svira u tihoj sobi. – Focus on the act of playing an instrument; still natural if we know he plays something.

If you want to be very clear about practicing an instrument, you can say:
Glazbenik često vježba klavir u tihoj sobi. – The musician often practices the piano in a quiet room.

How is the verb vježbati conjugated in the present tense?

Vježbati (to practice) present tense:

ja vježbam – I practice
ti vježbaš – you (sg.) practice
on/ona/ono vježba – he/she/it practices
mi vježbamo – we practice
vi vježbate – you (pl./formal) practice
oni/one/ona vježbaju – they practice

In the sentence, vježba is 3rd person singular: (on) vježba – he practices.

What case is tihoj sobi in, and why is it used here?

Tihoj sobi is in the locative singular case (feminine).

The preposition u (in) can be followed by:

Accusative – when there is movement into something (u sobu – into the room)
Locative – when something is located in something (u sobi – in the room)

Here we talk about location, not movement, so we use locative:

base form (nominative): tiha soba – quiet room
locative singular: u tihoj sobi – in a quiet room

Why is it tihoj sobi and not tiha soba? What changed?

The adjective and noun must agree in gender, number and case.

Base (nominative): tiha soba (quiet room) – feminine singular nominative
After u for location, you need locative: u tihoj sobi

Changes:

tiha → tihoj (feminine singular locative of tih / tiha / tiho)
soba → sobi (feminine singular locative of soba)

So both words change to locative singular feminine to match each other and the preposition u.

Can I say u tihom sobi or u tiha soba instead?

No, both are incorrect:

u tihom sobi – tihom is masculine/neuter form; sobi is feminine, so they do not agree.
u tiha soba – both are nominative; after u (for location) you need locative, not nominative.

Correct form is:

u tihoj sobi – adjective and noun both feminine singular locative.

What’s the difference between tihoj sobi and mirnoj sobi?

Both adjectives describe a room that is not noisy:

tiha soba / u tihoj sobi – quiet room (low volume, not loud)
mirna soba / u mirnoj sobi – calm, peaceful room (no disturbance, restful atmosphere)

They often overlap, but:

tiha focuses more on sound level.
mirna can also include lack of stress or activity, a peaceful feeling.

Both would be natural in this sentence, depending on what you want to emphasize.

Why is there no subject pronoun, like On često vježba?

Croatian is a pro-drop language: subject pronouns (ja, ti, on, ona…) are often omitted because verb endings show the person:

vježbam – must be I (ja)
vježba – could be he/she/it (on/ona/ono)

So Glazbenik često vježba… already tells us who is practicing.

You would say On često vježba when you need extra emphasis on he (as opposed to someone else), or if the subject is unclear from context.

How would I say Musicians often practice in quiet rooms (plural)?

You need plural forms of the noun, verb, adjective and noun in locative:

Glazbenici često vježbaju u tihim sobama.

Breakdown:

glazbenici – musicians (plural of glazbenik)
vježbaju – they practice (3rd person plural of vježbati)
tihim sobama – in quiet rooms (feminine plural locative of tihe sobe)

How do you pronounce često and vježba? What do č and ž sound like?

Approximate English equivalents:

često – /ˈtʃe.sto/
- č like ch in church
- stress on če: ČE-sto
vježba – /ˈvʲɛʒ.ba/
- vje sounds like vye
- ž like s in measure or vision
- stress on vjež: VJEŽ-ba

So:

često ≈ CHEH-sto
vježba ≈ VYEZH-ba (with a soft vye at the start)