Lektira joj je ostala u školi, pa ću je morati uzeti sutra.

Lektira is a school term. It usually means required reading or the book/text assigned for school reading.

So in this sentence, it does not just mean any book. It refers to the book or reading material she needs for school.

Joj is the dative singular form of ona and means to her / her.

In Croatian, a dative pronoun is often used to show whose something is, especially in situations like something being lost, left behind, or staying somewhere unexpectedly.

So:

• Lektira joj je ostala u školi
  literally: The required-reading book stayed to her at school
• natural English: She left her required-reading book at school

Using joj here is very natural. Croatian often prefers this kind of structure instead of explicitly saying njezina lektira.

Ostala is from the verb ostati, which usually means to remain, to stay, or to be left behind.

In this sentence, the most natural sense is was left behind.

So ostala u školi means something like:

• stayed at school
• was left at school
• got left behind at school

Because ostala agrees with lektira, and lektira is:

• feminine
• singular

So the past participle must also be feminine singular:

• masculine singular: ostao
• feminine singular: ostala
• neuter singular: ostalo

Since lektira is feminine, ostala is the correct form.

That je is the auxiliary verb used to form the perfect tense.

So:

• ostala = past participle
• je ostala = has remained / stayed / was left

In Croatian, this is a normal past-tense structure.

U školi means in school / at school.

Here u expresses location, so škola goes into the locative case:

• nominative: škola
• locative: školi

So:

• u školu = into the school (movement, accusative)
• u školi = in the school / at school (location, locative)

This sentence is about where the book was left, so u školi is correct.

Pa here means so, and so, or therefore.

It connects the two ideas:

• her required-reading book was left at school
• as a result, I’ll have to get it tomorrow

So it works a bit like so in English.

This je is not the auxiliary verb. Here it is the accusative singular pronoun meaning it.

It refers back to lektira.

So:

• first je = auxiliary in je ostala
• second je = object pronoun it

That can be confusing at first, because the forms look identical.

This part breaks down as:

• ću = I will
• je = it
• morati = have to
• uzeti = take / get / pick up

So the whole phrase means:

• I will have to take/get it
• more naturally here: I’ll have to pick it up tomorrow

This is basically the Croatian equivalent of will have to + verb in English.

Yes, Croatian word order is fairly flexible, but small unstressed words like ću, je, and joj are clitics, and they usually go near the beginning of the clause.

That is why you get structures like:

• Lektira joj je ostala...
• pa ću je morati uzeti...

Other word orders are also possible, for example:

• Morat ću je uzeti sutra.
• Sutra ću je morati uzeti.

These all mean roughly the same thing, but the emphasis shifts slightly.

Literally, yes, uzeti means to take.

But in this context, a more natural English translation is often:

• pick up
• go get
• retrieve

So morati uzeti sutra is most naturally understood as have to pick it up tomorrow.

Sutra means tomorrow.

Putting it at the end is completely normal and sounds natural. Croatian often places time expressions like this at the end unless the speaker wants to emphasize them.

For example:

• Pa ću je morati uzeti sutra. = neutral
• Sutra ću je morati uzeti. = slightly more emphasis on tomorrow