Kad budem u gradu, vratit ću knjige u knjižnicu.

In Croatian, after time words like kad/kada (when), you normally do not use ordinary future tense the way English learners might expect.

So Croatian says:

• Kad budem u gradu, ...

not usually:

• Kad ću biti u gradu, ...

The form kad ću... sounds more like a direct question such as When will I...?, not a normal time clause.

A good learner rule is:

• main future clause: use future I
• future time clause with kad/ako/čim: often use budem or another non-future-I form

So this sentence is standard Croatian.

Budem is the form Croatian uses with biti in this kind of future-dependent clause.

In practical terms, it means something like:

• when I am
• once I’m
• when I happen to be

So Kad budem u gradu means When I’m in town / When I’m in the city.

For a learner, the important thing is not to over-translate it word for word. Just recognize budem as the normal Croatian form used after kad when the situation is in the future.

Croatian future I is formed with:

• the infinitive
• plus the short form of htjeti: ću, ćeš, će, ćemo, ćete, će

With vratiti, you can say:

• Vratit ću knjige.
• Ja ću vratiti knjige.

Both are correct.

When the auxiliary comes after the infinitive, standard Croatian drops the final -i of the infinitive in writing:

• vratiti + ću → vratit ću

So:

• vratit ću = correct
• vratiti ću = non-standard

Because Croatian uses different cases depending on whether you mean:

• location: being somewhere
• direction: going/moving somewhere

Here:

• u gradu = in the city/town → location
  so u + locative
• u knjižnicu = to/into the library → direction
  so u + accusative

That contrast is very important in Croatian:

• u gradu = in town
• u grad = into town / to town

and

• u knjižnici = in the library
• u knjižnicu = to the library

Here knjige is the direct object, so it is in the accusative plural.

The verb vratiti takes a direct object:

• return what? → knjige

For this noun, the nominative plural and accusative plural happen to look the same:

• nominative plural: knjige
• accusative plural: knjige

So the form itself does not change, but its function in the sentence is accusative.

Yes. Vratiti is perfective.

That means it presents the action as:

• completed
• whole
• finished

That fits this sentence well, because the speaker means a single completed act: they will return the books.

If you used the imperfective vraćati, it would usually suggest something like:

• repeated action
• habitual action
• ongoing process

So:

• vratit ću knjige = I’ll return the books (as a completed act)
• vraćat ću knjige = I’ll be returning books / I’ll return books repeatedly, depending on context

Yes, Croatian word order is more flexible than English word order.

The original sentence is a natural, neutral version:

• Kad budem u gradu, vratit ću knjige u knjižnicu.

But you could also say, for example:

• Kad budem u gradu, knjige ću vratiti u knjižnicu.
• Knjige ću vratiti u knjižnicu kad budem u gradu.

The meaning stays very similar, but the emphasis changes a little.

One thing to watch out for is the clitic ću. Croatian clitics tend to go in the second position of their clause, so word order is flexible, but not completely free.

Yes, in most cases they mean the same thing: when.

So both are possible:

• Kad budem u gradu...
• Kada budem u gradu...

The difference is mostly stylistic:

• kad = very common, everyday, slightly shorter
• kada = a bit fuller, sometimes a little more formal or careful

In this sentence, kad sounds completely natural.

Because Kad budem u gradu is a subordinate clause placed before the main clause.

Croatian normally separates that kind of introductory clause with a comma:

• Kad budem u gradu, vratit ću knjige u knjižnicu.

If the order is reversed, the comma is usually not needed in the same way:

• Vratit ću knjige u knjižnicu kad budem u gradu.

So the comma here is standard punctuation.

Croatian has no articles, so there is no direct equivalent of English a/an/the.

That means:

• knjige can mean books or the books
• knjižnicu can mean a library or the library

Context usually makes it clear.

If you want to sound more specific or emphatic, Croatian can use demonstratives such as:

• te knjige = those / the specific books
• tu knjižnicu = that / the specific library

But in a normal sentence like this, no article is needed.

Yes. Grad often covers both city and town, depending on context.

So:

• u gradu may mean in the city
• or in town

That is why translations may vary. Croatian grad is broader than English city in some contexts, especially in everyday speech.