Želim probati plavu haljinu u kabini.

Because after želim (I want) Croatian normally uses the infinitive of the second verb.

• želim probati = I want to try
• not želim probam

So the pattern is:

• želim + infinitive
• moram + infinitive
• mogu + infinitive

Examples:

• Želim kupiti haljinu. = I want to buy a dress.
• Mogu probati ovu? = Can I try this one?

Because plavu haljinu is in the accusative case, not the nominative.

The dress is the direct object of probati (to try on / try), so both the adjective and the noun change form.

• nominative: plava haljina = a blue dress
• accusative: plavu haljinu = a blue dress as the thing being tried

This is very common in Croatian:

• Vidim plavu haljinu.
• Kupujem plavu haljinu.
• Želim probati plavu haljinu.

In all of these, the dress is the object, so accusative is used.

Because in Croatian, adjectives must agree with the noun they describe in:

• gender
• number
• case

Here, haljina is:

• feminine
• singular
• accusative

So the adjective must match it:

• plava haljina = nominative feminine singular
• plavu haljinu = accusative feminine singular

This agreement is one of the most important patterns in Croatian grammar.

Because u can take different cases depending on meaning.

Here, u kabini means in the fitting room / inside the fitting room, so it shows location, and Croatian uses the locative.

• u kabini = in the fitting room

If you were talking about movement into the fitting room, you would use the accusative:

• Idem u kabinu. = I am going into the fitting room.

So:

• location → u + locative
• destination / movement into → u + accusative

Yes, in a clothing-store context, kabina commonly means a changing room / fitting room.

You may also hear:

• kabina za presvlačenje = changing room
• garderoba in some contexts, though that can mean other things too

In everyday shopping Croatian, u kabini is a natural way to say in the fitting room.

Because Croatian often drops subject pronouns when they are already clear from the verb form.

• Želim already means I want
• so ja is not necessary

You could say:

• Ja želim probati plavu haljinu u kabini.

But that usually adds emphasis, like I want to try it on.

This is normal in Croatian:

• Kupujem. = I am buying
• Idemo. = We are going
• Možeš li pomoći? = Can you help?

Yes. Croatian word order is more flexible than English word order because cases show the grammatical roles.

The most neutral order here is:

• Želim probati plavu haljinu u kabini.

But other orders are possible for emphasis, for example:

• Plavu haljinu želim probati u kabini.
  Emphasizes the blue dress.
• U kabini želim probati plavu haljinu.
  Emphasizes in the fitting room.

Even though the order can change, the most natural choice in a shop is usually the original sentence.

The dictionary form is haljina.

Croatian dictionaries usually list nouns in the nominative singular form, so you would look up:

• haljina = dress

In the sentence, it appears as haljinu because it is in the accusative singular.

So:

• dictionary form: haljina
• sentence form: haljinu

Yes, both are possible.

• probati = to try
• isprobati = often to try out / try on, sometimes slightly more explicit or complete

In a shop, people commonly say both:

• Želim probati ovu haljinu.
• Želim isprobati ovu haljinu.

In everyday speech, probati is very common and completely natural.

Not always. Plav / plava / plavo can mean:

• blue for things
• blond / fair-haired for people, depending on context

But with haljina (dress), it clearly means blue:

• plava haljina = blue dress

If it described a woman, context could point to blond:

• plava djevojka = usually a blond girl

So the meaning depends on the noun and the situation.