Learn Hardcore Croatian: Na prozoru je tanka plava zavjesa, a iza nje je bijeli zid. - On the window there is a thin blue curtain, and behind it there is a white wall.

Accusative = motion onto something
- Stavljam zavjesu na prozor. – I’m putting the curtain onto the window.
Locative = location on something (no movement)
- Na prozoru je zavjesa. – The curtain is on the window.

In your sentence, it’s describing position (where the curtain is), not movement, so na uses the locative case → prozoru.

So:

na prozoru = on the window (location → locative)
na prozor = onto the window (movement → accusative)

What is the case and form of prozoru, and what is the base form?

The base (dictionary) form is prozor (window), masculine singular nominative.

Its singular declension (key cases) looks like this:

Nominative: prozor – the window (subject)
Genitive: prozora – of the window
Dative/Locative: prozoru – to the window / on the window / at the window
Accusative: prozor – (onto) the window
Instrumental: prozorem – with the window (less common)

In na prozoru, prozoru is locative singular, required after na when it means “on” in the static sense.

Why does the sentence start with Na prozoru instead of Tanka plava zavjesa je na prozoru?

Both orders are grammatically correct:

Na prozoru je tanka plava zavjesa.
Tanka plava zavjesa je na prozoru.

The difference is in focus and style:

Na prozoru je tanka plava zavjesa.
– First sets the scene/location (“On the window...”), then says what is there. This is very natural in descriptions.
Tanka plava zavjesa je na prozoru.
– First focuses on which thing you’re talking about (“The thin blue curtain...”), then says where it is. This can be used if the curtain is already the topic in the conversation.

Croatian word order is flexible, but the clitic je usually wants to be in the second position in the clause, which is why we get:

[Na prozoru] je [tanka plava zavjesa].
[Tanka plava zavjesa] je [na prozoru].

Why are the adjectives tanka and plava in that exact form?

They must agree with the noun zavjesa in:

gender: feminine
number: singular
case: nominative (subject of the verb je)

So:

zavjesa – feminine singular nominative
tanka – feminine singular nominative of tanak (thin)
plava – feminine singular nominative of plav (blue)

That’s why it’s tanka plava zavjesa, not tanak plav zavjesa or similar.

Is there any rule about the order tanka plava (thin blue), or could it be plava tanka?

Both tanka plava zavjesa and plava tanka zavjesa are grammatically possible, but the first is more natural.

Croatian doesn’t have a very rigid adjective order like English, but there are tendencies:

more “descriptive” / physical properties (size, shape, thickness) often come before
more “inherent” or more easily visualized qualities like color often come later

So:

tanka plava zavjesa
– sounds like “a thin, blue curtain” (normal, neutral order)
plava tanka zavjesa
– sounds a bit unusual, can be used for emphasis on blue or in poetic style.

In everyday speech, tanka plava zavjesa is preferred.

What exactly does the conjunction a mean here, and how is it different from i?

In Croatian, both a and i can translate as “and”, but they are used differently:

i = simple addition, just linking items or clauses
- Na prozoru je zavjesa i iza nje je zid.
a = “and / while / whereas / but” – often marks a contrast, change of scene, or shift in topic

In your sentence:

Na prozoru je tanka plava zavjesa, a iza nje je bijeli zid.

a suggests a slight shift: first you describe what is on the window, then what is behind it. It’s not a strong “but”, more like “and meanwhile / and then” with a sense of contrast between foreground and background.

Using i here would be grammatical, but a sounds more natural stylistically.

Why is it iza nje and not iza nju?

The preposition iza (behind) takes the genitive case:

iza + genitive = behind (static location)

The pronoun for “her/it” (referring to zavjesa, which is feminine) in the genitive is nje.

Forms of ona (she / that feminine thing):

Nominative: ona (she)
Genitive: nje (of her / of it)
Dative/Locative: njoj
Accusative: nju
Instrumental: njom

So:

iza nje – behind her / behind it (genitive, correct after iza)
iza nju – wrong case; nju is accusative, not genitive

What does nje refer to exactly? Why not repeat zavjesa?

Nje is the genitive singular feminine pronoun, standing for zavjesa.

So:

iza nje literally means “behind her/it” → “behind the curtain”.

You could repeat the noun:

… a iza zavjese je bijeli zid. – “…and behind the curtain there is a white wall.”

Both are correct. Using nje avoids repetition and sounds natural after zavjesa has just been mentioned.

Why is it bijeli zid and not beli zid?

This is a standard Croatian vs. Serbian (and some regional) difference:

Standard Croatian: bijeli (white)
Standard Serbian (ekavian): beli (white)

In Croatian, the adjective is bijel in its basic form, and:

Masculine singular nominative (attributive long form): bijeli zid – a white wall

So in standard Croatian you say bijeli zid, not beli zid.

Why is the form bijeli used and not just bijel?

Croatian adjectives have short and long forms. For many common adjectives, the long form is used most often in front of a noun:

bijeli zid – a white wall (normal, everyday form)
bijel zid – feels poetic, archaic, or stylistically marked

So the usual, neutral way to say it is bijeli zid.

What case is zid in, and why?

Zid (wall) is in the nominative singular:

It is the subject of the second clause: iza nje je bijeli zid.
zid is what exists/is behind the curtain.

Short analysis:

iza nje – prepositional phrase (behind it)
je – verb “is”
bijeli zid – subject (what is there)

So zid must be nominative to serve as the subject.

Why is je in the middle (after Na prozoru / after iza nje) and not at the start of the clause?

The verb je (is) is a clitic. In Croatian, clitics usually move to the second position in the clause (Wackernagel position).

That’s why the word order is:

Na prozoru je tanka plava zavjesa.
- 1st element: Na prozoru
- 2nd (clitic) position: je
Iza nje je bijeli zid.
- 1st element: Iza nje
- 2nd (clitic) position: je

You cannot put je first:
Je na prozoru tanka plava zavjesa sounds wrong in standard Croatian.

How does je express the English idea of “there is / there are” here?

Croatian often uses a simple “to be” sentence to express existence or “there is/are”:

Na prozoru je tanka plava zavjesa.
Literally: “On the window is a thin blue curtain.”
Meaning: “There is a thin blue curtain on the window.”
Iza nje je bijeli zid.
Literally: “Behind it is a white wall.”
Meaning: “There is a white wall behind it.”

So je (is) functions like “there is” in this type of structure.

Is the comma before a necessary?

Yes, in Croatian the comma is normally used before a when it joins two independent clauses:

Na prozoru je tanka plava zavjesa, a iza nje je bijeli zid.

Each part could stand alone as a sentence:

Na prozoru je tanka plava zavjesa.
Iza nje je bijeli zid.

Because a links two full clauses, the comma is required in standard writing.

Could I say a iza zavjese je bijeli zid instead of a iza nje je bijeli zid? Is there any difference?

Yes, you can say:

Na prozoru je tanka plava zavjesa, a iza zavjese je bijeli zid.

Both versions are correct:

iza nje – behind it (pronoun referring to zavjesa)
iza zavjese – behind the curtain (noun repeated)

The difference is stylistic:

iza nje: slightly lighter, avoids repetition, sounds more fluent in context.
iza zavjese: more explicit; you repeat the noun instead of using a pronoun.

In natural speech, using nje here is very common once zavjesa has already been mentioned.