U izlogu vidim posteljinu iste boje kao naše zavjese.

Because u can take different cases depending on meaning:

• u + locative = location, in / inside / in the area of
• u + accusative = motion into

Here the meaning is location: the bedding is in the shop window / display window, so Croatian uses the locative:

• izlog → u izlogu

So u izlogu means in the display window.

Izlog usually means a shop window or display window, not just any glass window of a building.

So u izlogu here means something like:

• in the shop window
• in the display
• in the window display

Because posteljinu is the accusative singular form, and it is the direct object of vidim (I see).

The noun is:

• nominative: posteljina
• accusative: posteljinu

Since the sentence says I see bedding, Croatian uses the accusative:

• Vidim posteljinu.

Grammatically, it is singular, but its meaning is often collective, like English bedding or bed linen.

So posteljina can refer to a whole set of bedding items, even though the noun itself is singular.

That is why Croatian says:

• vidim posteljinu

not necessarily a plural form.

Because Croatian often expresses this idea as of the same color.

So:

• ista boja = the same color
• iste boje = of the same color

In this sentence, the bedding is being described by a phrase equivalent to bedding of the same color as our curtains, so Croatian uses the genitive:

• boja → boje
• ista → iste

Hence:

• posteljinu iste boje

Iste agrees with boje, not with posteljinu.

Here is the structure:

• boje = genitive singular of boja
• iste matches it in gender, number, and case

So:

• iste boje = of the same color

Even though posteljinu is also feminine, iste is not describing posteljinu directly here. It belongs to the phrase built around boje.

Because kao here introduces a comparison: as / like our curtains.

The phrase means:

• the same color as our curtains

You can think of it as a shortened version of something like:

• iste boje kao što su naše zavjese

So kao does not force the genitive here. That is why naše zavjese is correct.

In this sentence, it is best understood as nominative plural in an elliptical comparison after kao.

A useful thing to know is that for many feminine plural nouns, the nominative plural and accusative plural forms look the same. So:

• zavjese can look identical in both cases

But here, because of the comparison structure with kao, nominative is the most natural explanation.

Also:

• naše agrees with zavjese
• feminine plural: naše zavjese = our curtains

Yes. Croatian word order is fairly flexible because case endings show grammatical roles.

This version is natural:

• U izlogu vidim posteljinu iste boje kao naše zavjese.

But you could also say:

• Vidim u izlogu posteljinu iste boje kao naše zavjese.

Both are grammatical. The difference is mostly about focus and style:

• starting with U izlogu highlights the location first
• starting with Vidim highlights the action I see

Because vidim means I see, while gledam means I look at / I am watching.

So:

• vidim = I notice or perceive something
• gledam = I am deliberately looking at something

In a sentence like this, the speaker is saying that they see bedding in the display window, so vidim is the natural choice.

Because Croatian does not have articles like English a and the.

So posteljinu can mean:

• the bedding
• some bedding
• bedding

The exact meaning depends on context, not on an article.