Otkud ti ta plava suknja?

They are very close, and in many situations either one works.

• odakle = more straightforward from where
• otkud = also from where, but it often sounds a bit more conversational and can carry a shade of surprise or how come

So Odakle ti ta plava suknja? is also natural, but Otkud... can feel a little more like Where on earth did you get... ?

ti is the unstressed dative form of you (singular, informal). It does not literally mean your, but in this kind of Croatian sentence it helps show whose item we are talking about.

So Otkud ti ta plava suknja? is literally something like:

From where to-you that blue skirt?

But idiomatically it means:

Where did you get that blue skirt?
or
How come you have that blue skirt?

If you want to emphasize you, Croatian often uses tebi instead:

Otkud tebi ta plava suknja?

Because in this sentence ta plava suknja is in the nominative, not the accusative.

This question is a short, elliptical Croatian structure with no explicit verb like buy or get. Because of that, the skirt phrase stays in nominative:

• ta plava suknja = nominative

If you used a full verb such as kupiti (to buy), then you would use the accusative:

• Gdje si kupila tu plavu suknju? = Where did you buy that blue skirt?

So the difference comes from the sentence structure.

Because they agree with suknja, which is:

• feminine
• singular
• nominative

Croatian adjectives and demonstratives must agree with the noun in gender, number, and case:

• ta = feminine singular nominative of taj
• plava = feminine singular nominative of plav
• suknja = feminine singular nominative noun

That is why all three forms match.

ta is a demonstrative, meaning that. It points to a specific skirt that is visible, known, or already being talked about.

Croatian has no articles like a or the, so learners sometimes wonder whether ta is acting like the. It is not an article; it is more specific than that. It means something like:

• that blue skirt
• sometimes, in natural English, just the blue skirt, if the context already makes it clear

Because ti is a clitic. Croatian clitics usually go in second position in the sentence.

Here the first stressed word is Otkud, so the clitic ti comes right after it:

• Otkud ti ta plava suknja?

This is a very typical Croatian pattern. The full noun phrase ta plava suknja comes after the clitic.

Yes, in a way. This is a very natural elliptical spoken Croatian question, where the verb is not stated explicitly.

English usually needs to insert a verb such as:

• get
• have
• sometimes buy, depending on context

So Croatian says:

• Otkud ti ta plava suknja?

But English naturally says:

• Where did you get that blue skirt?
• How come you have that blue skirt?

Croatian often leaves that verb implied when the meaning is obvious.

Yes. plav / plava / plavo can mean both blue and blond(e), depending on context.

In this sentence it definitely means blue, because it describes suknja (skirt), not a person’s hair.

So:

• plava suknja = blue skirt
• plava djevojka could mean a blonde girl

It is very natural and conversational. Depending on intonation, it can sound like:

• simple curiosity
• admiration
• surprise
• suspicion
• even mild disapproval

For example, with surprised intonation, it can feel like:

Where did you get that blue skirt?

with an implied I didn’t expect to see you with that.

You have several options, depending on what exactly you mean:

• Gdje si kupila tu plavu suknju? = Where did you buy that blue skirt?
• Gdje si nabavila tu plavu suknju? = Where did you get that blue skirt?
• Kako to da imaš tu plavu suknju? = How come you have that blue skirt?

So Otkud ti ta plava suknja? is shorter and more conversational, while these versions are more explicit.