Iako trenutno nemam nikakav veliki plan za vikend, vjerojatno ću svratiti do sestre i donijeti joj knjige koje sam joj već obećala.

Iako means although or even though. It introduces a subordinate clause:

Iako trenutno nemam nikakav veliki plan za vikend
= Although I currently do not have any big plan for the weekend

In Croatian, when this kind of subordinate clause comes first, it is normally separated from the main clause by a comma.

So the structure is:

Iako ... , vjerojatno ću ...

Trenutno means currently, at the moment, or right now.

Here it modifies nemam:

trenutno nemam
= I currently do not have

It gives the idea that this is the speaker’s situation now, not necessarily forever.

Nikakav adds emphasis. It often means something like:

• no kind of
• no particular
• no real

So:

nemam nikakav veliki plan
is stronger than simply
nemam veliki plan

It suggests something like:

• I don’t have any big plan
• I don’t really have any major plan
• I have no particular big plan

Also, grammatically, nikakav veliki plan is the direct object of nemam.

Za vikend is a very common Croatian time expression meaning:

• for the weekend
• this weekend
• at the weekend

The preposition za is often used in time expressions like this.

Here, vikend is in the accusative, but because it is masculine inanimate, its accusative singular looks the same as the nominative:

• nominative: vikend
• accusative: vikend

So you see no form change.

This is the Croatian future tense, called future I.

It is formed with:

• a form of htjeti used as an auxiliary
• plus the infinitive

Here:

• ću = I will
• svratiti = infinitive, to drop by

So:

vjerojatno ću svratiti
= I will probably drop by

Why is ću there? Because ću is a clitic, and clitics usually go in the second position in the clause. Since vjerojatno comes first, ću comes right after it.

You can also often put the infinitive first in Croatian future, but then the infinitive is normally shortened:

• svratit ću

Standard Croatian does not normally use svratiti ću.

Svratiti means to drop by, to stop in briefly, or to pop over.

It usually suggests a short, casual visit.

Do sestre literally means to the sister, but idiomatically it means to my sister’s place or to see my sister.

The preposition do takes the genitive, so:

• sestra = nominative
• sestre = genitive singular

So:

svratiti do sestre
= to drop by my sister’s place

This is a very natural Croatian way to express a brief visit.

Here:

• donijeti = to bring
• joj = to her
• knjige = books

So the structure is:

donijeti joj knjige
= bring her the books

Joj is the dative singular form of ona. It marks the recipient, the person who receives something.

So Croatian uses:

• direct object: knjige
• indirect object: joj

Just like English:

• bring the books to her
• bring her the books

Because it belongs to two different verbs.

First:

donijeti joj knjige
= bring her the books

Second:

koje sam joj već obećala
= that I already promised her

So each verb has its own joj:

• donijeti komu? → joj
• obećati komu? → joj

English sometimes avoids repeating her, but Croatian naturally repeats it when each verb requires it.

Koje refers back to knjige and means that or which.

So:

knjige koje sam joj već obećala
= the books that I already promised her

The relative pronoun must match its antecedent, knjige, in gender and number:

• knjige = feminine plural

But its case depends on its role inside the relative clause. Here it is the object of obećala, so it is in the accusative plural.

For feminine plural, nominative and accusative have the same form:

• koje

Because the sentence shows that the speaker is female.

In Croatian past tense, the active participle agrees with the subject’s gender:

• obećao = a male speaker
• obećala = a female speaker

So:

sam joj već obećala
means I already promised her, said by a woman.

If the speaker were male, it would be:

sam joj već obećao

Because the promise happened before the future action of bringing the books.

The timeline is:

1. the speaker already promised the books
2. now she is talking about the weekend
3. she will probably bring them then

So Croatian uses the past tense here very naturally:

koje sam joj već obećala
= which I already promised her

The word već strengthens that idea of already.

Both verbs describe single, completed actions that the speaker expects to do:

• svratiti = drop by once
• donijeti = bring something and complete the action

That is why perfective verbs are natural here. In Croatian, perfective verbs are very common in the future when you mean a one-time completed event.

Using imperfective verbs would change the meaning and could suggest repeated action, ongoing action, or a different viewpoint.

So this sentence sounds natural because the speaker is talking about likely, concrete actions on the weekend.