Nađe li knjižničarka drugi primjerak lektire, ostavit će ga za mog sina do sutra.

This is a standard Croatian pattern:

• verb + li can introduce a conditional clause
• so Nađe li knjižničarka... means If the librarian finds...

It is very close in meaning to:

• Ako knjižničarka nađe...

The verb + li version is a bit more formal, compact, and somewhat more typical of written style. A learner will often first meet ako for if, but this nađe li pattern is very common and worth recognizing.

Because in this construction, the verb normally comes first:

• Nađe li knjižničarka...

The particle li attaches right after the verb, so the subject usually follows it. That word order is part of the pattern.

Compare:

• Ako knjižničarka nađe...
• Nađe li knjižničarka...

Both are correct, but the second one naturally begins with the verb.

In Croatian, future conditions often use the present tense in the if-clause, especially with a perfective verb.

So:

• nađe = present form of naći
• but in a sentence like this it means if she finds in the future

Then the main clause uses the future:

• ostavit će = she will leave / keep

This is normal Croatian grammar:

• Ako dođe, nazvat ću te. = If he/she comes, I’ll call you.
• Nađe li... , ostavit će...

Nađe is the 3rd person singular present of the verb naći (to find).

So:

• ja nađem = I find
• ti nađeš = you find
• on/ona nađe = he/she finds

Here it agrees with knjižničarka, which is singular and feminine, so in English we translate it as she finds.

Also, naći is a perfective verb, which is why the action is understood as a completed finding: she manages to find a copy.

This is the standard way Croatian writes one type of future tense formation.

The verb is ostaviti (to leave, put aside, keep for someone). In the future:

• ostaviti će is not the standard written form
• standard spelling is ostavit će

What happens is:

• the infinitive loses final -i? No
• more precisely, the infinitive ending -ti is reduced before će
• ostaviti + će becomes ostavit će

Compare:

• Ona će ostaviti knjigu.
• Ostavit će knjigu.

Both mean She will leave the book, but the word order is different.

Ga is the short unstressed pronoun meaning him/it in the accusative singular.

Here it refers to:

• drugi primjerak = another/second copy

Since primjerak is masculine singular, the pronoun is:

• ga = it

So:

• ostavit će ga = she will leave/keep it

Croatian often uses these short clitic pronouns instead of repeating the noun.

Because ga is the normal short form used when there is no special emphasis.

• ga = unstressed clitic form
• njega = full stressed form

In this sentence, there is no contrast or emphasis, so ga is the natural choice.

You would use njega if you wanted emphasis, for example:

• Neću uzeti prvi primjerak, nego njega.
  = I won’t take the first copy, but that one.

Also, after a preposition, Croatian uses the full form, not the short one.

Drugi primjerak lektire is the direct object of nađe, so:

• drugi primjerak is in the accusative singular

Because primjerak is masculine inanimate, its accusative looks the same as the nominative:

• primjerak → primjerak

Then lektire is in the genitive singular because it depends on primjerak:

• primjerak lektire = a copy of the required reading

So the structure is:

• drugi primjerak = another/second copy
• lektire = of the reading / of the required text

Lektira usually means required reading, especially school reading assigned to students.

So primjerak lektire is not just any book in general, but specifically:

• a copy of the assigned reading
• a copy of the required school text/book

That is why the librarian is looking for another copy for the speaker’s son.

Because the preposition za here means for, and in this meaning it takes the accusative.

So:

• moj sin = nominative
• mog sina = accusative

Since sin is a masculine animate noun, its accusative matches the genitive form:

• sin → sina

And the possessive adjective changes too:

• moj → mog (also often moga in some styles)

So:

• za mog sina = for my son

It can suggest either, depending on context, but here the most natural sense is:

• until tomorrow

So the idea is that the librarian will keep the copy aside for the speaker’s son up to tomorrow.

Croatian do + genitive often marks an endpoint in time:

• do večeras = until this evening
• do sutra = until tomorrow

In some contexts English may also express this as by tomorrow, but in this sentence until tomorrow is probably the better interpretation.

It can mean either, depending on context.

• drugi literally means second
• but in many everyday contexts it can also mean another

So drugi primjerak lektire may mean:

• the second copy of the required reading, or
• another copy of the required reading

If the meaning shown to the learner is another copy, that is perfectly natural here.

Because Croatian does not have articles like English a/an and the.

So:

• knjižničarka can mean a librarian or the librarian
• the exact meaning comes from context

The same is true throughout the sentence. Croatian relies on context, word order, and shared knowledge instead of articles.

So an English speaker should not expect a separate word corresponding to the.