Poklon za rođendan je već kupljen.

Here is the role of each part:

• poklon = gift, present
  It is the main noun and the subject of the sentence.

• za = for
  This preposition is used here in the sense of purpose or intended occasion.

• rođendan = birthday
  Together with za, it makes za rođendan = for a birthday / for the birthday.

• je = is
  This is the 3rd person singular present form of biti (to be).

• već = already

• kupljen = bought
  More exactly, it is a passive past participle meaning bought in the sense has been bought / is bought already.

So the structure is basically:

[The birthday gift] [is] [already] [bought].

Because the preposition za normally takes the accusative case when it means for.

So:

• za + accusative

The noun rođendan is a masculine inanimate noun. In the singular, masculine inanimate nouns often have the same form in the nominative and accusative.

That means:

• nominative: rođendan
• accusative: rođendan

So although the form looks unchanged, it is still accusative here because it comes after za.

This is a very common point for English speakers.

In Croatian, many masculine inanimate singular nouns have:

• nominative = accusative

So rođendan does not visibly change.

Compare:

• Vidim stol. = I see a table.
• stol is accusative here, but it looks the same as nominative.

So in za rođendan, the case has changed grammatically, even though the word form has not.

Je kupljen is a passive-style construction, focusing on the gift, not on who bought it.

So the sentence emphasizes the result:

• The gift is already bought.
• more naturally in English: The birthday present has already been bought.

If you wanted an active version, you could say:

• Netko je već kupio poklon za rođendan.
  = Someone has already bought the birthday present.

Croatian often uses this passive/resultative type of expression when the important thing is that the action is done, not who did it.

Kupljen is the passive past participle of the verb kupiti (to buy).

It agrees with the noun it describes:

• masculine singular: kupljen
• feminine singular: kupljena
• neuter singular: kupljeno
• masculine personal plural: kupljeni
• feminine plural: kupljene
• neuter plural: kupljena

Since poklon is masculine singular, the sentence uses kupljen.

Because kupljen must agree with poklon.

• poklon is masculine
• singular
• nominative, because it is the subject

So the participle must also be:

• masculine singular nominative → kupljen

If the noun changed, the participle would change too:

• Knjiga je već kupljena. = The book has already been bought.
• Pismo je već kupljeno. = The letter has already been bought/written?
  Grammatically possible, though semantically odd.
• Darovi su već kupljeni. = The gifts have already been bought.

Formally, it uses the present tense of biti (je) plus a participle, but the meaning is often resultative:

• the action of buying happened earlier
• the result is true now

That is why in English it is often best translated as:

• The birthday present has already been bought.

Depending on context, English might also say:

• The birthday present is already bought.

So Croatian is expressing a current state resulting from a completed action.

Već means already.

In this sentence, it comes between je and kupljen:

• Poklon za rođendan je već kupljen.

That is a very natural position.

Croatian word order is flexible, so you may also see variations, for example:

• Poklon za rođendan već je kupljen.
• Već je kupljen poklon za rođendan.

These can all be correct, but the original sentence is neutral and very common.

Yes, that is grammatically possible, but it sounds a little different in focus.

• Poklon za rođendan je već kupljen.
  This treats poklon za rođendan as one unit: the birthday gift.

• Poklon je već kupljen za rođendan.
  This can sound more like The gift has already been bought for the birthday, where za rođendan is attached a bit later as extra information.

The original version is more natural if birthday gift is the intended noun phrase.

Yes.

The core noun is poklon, and za rođendan modifies it, telling us what kind of gift it is:

• poklon = gift
• poklon za rođendan = birthday gift / gift for a birthday

So the whole noun phrase functions as the subject of the sentence.

Croatian has no articles.

English distinguishes between:

• a gift
• the gift

Croatian does not do that with separate words. Instead, definiteness is understood from:

• context
• word order
• intonation
• the situation

So poklon can mean:

• a gift
• the gift

In this sentence, English will usually translate it as the birthday present because the context often suggests a specific gift.

Yes.

Because Croatian has no articles, za rođendan can be interpreted as:

• for a birthday
• for the birthday
• sometimes more idiomatically, as a birthday gift

Context decides which is most natural in English.

Yes, but then the structure changes and you need a subject that actually does the buying.

For example:

• Kupio sam poklon za rođendan.
  = I bought the birthday gift.

• Netko je već kupio poklon za rođendan.
  = Someone has already bought the birthday gift.

You cannot simply replace kupljen with kupio in the original sentence, because:

• kupljen describes the gift
• kupio describes the person who bought it

So they are used in different sentence patterns.

In standard Croatian, yes.

The sentence needs the auxiliary je to form this passive/resultative construction:

• Poklon za rođendan je već kupljen.

Without je, the sentence would sound incomplete in normal standard usage.

Yes, very natural.

It sounds like something someone would say when discussing party or birthday preparations:

• Do we still need to buy a present?
• No, poklon za rođendan je već kupljen.

It is a standard, everyday way to say that the present has already been taken care of.

A very natural active equivalent is:

• Već smo kupili poklon za rođendan.
  = We’ve already bought the birthday present.

Or, if the buyer is unknown or unimportant:

• Netko je već kupio poklon za rođendan.
  = Someone has already bought the birthday present.

The passive version keeps the focus on the present, while the active version focuses more on who did the buying.