Moj sin troši džeparac na stripove, a moja kći ga radije čuva za knjige.

Because moj/moja must agree with the noun they describe.

• sin is masculine singular, so you use moj
• kći is feminine singular, so you use moja

So:

• moj sin = my son
• moja kći = my daughter

These possessive words behave like adjectives in Croatian, so they change for gender, number, and case.

Croatian normally does not use articles.

So:

• sin can mean a son, the son, or just son, depending on context
• džeparac can mean pocket money or the pocket money

English needs articles, but Croatian usually lets the context do that job.

Here a connects two clauses and shows a contrast:

• Moj sin troši...
• a moja kći...

In English, this can sound like:

• and
• while
• whereas
• but

It is softer than ali (but in a stronger, more direct sense).
So here a suggests something like:

My son spends his pocket money on comics, whereas my daughter prefers to save it for books.

Because the verb trošiti (to spend) commonly goes with na + accusative when you say what something is spent on.

So:

• trošiti novac na nešto = to spend money on something
• troši džeparac na stripove = he spends his pocket money on comics

The preposition na here is not a literal on in a physical sense. It is just the normal pattern used with trošiti.

Because čuvati za means to keep/save for a purpose.

So:

• čuvati novac za knjige = to save money for books
• čuvati za putovanje = to save for a trip

So the two verbs use different prepositional patterns:

• trošiti na + accusative = spend on
• čuvati za + accusative = save for

That is why the sentence has:

• na stripove
• za knjige

Both are in the accusative plural, because they come after prepositions that require the accusative here.

stripove

The basic singular is strip.
Its plural forms include:

• stripovi = nominative plural
• stripove = accusative plural

So after na, you get:

• na stripove

knjige

The singular is knjiga.
The plural is:

• knjige = nominative plural
• knjige = accusative plural

So after za, you also get:

• za knjige

For many feminine nouns like knjiga, nominative plural and accusative plural are the same.

Ga refers back to džeparac.

So:

• džeparac = pocket money
• ga = it

More specifically, ga is the unstressed accusative singular form of the third-person pronoun used for a masculine noun here.

So the second clause literally means something like:

• my daughter would rather save it for books

instead of repeating džeparac again.

Because ga is a clitic: a short, unstressed word that has a special position in Croatian.

Clitics usually come near the beginning of the clause, often in second position.

So in:

a moja kći ga radije čuva za knjige

the opening phrase is moja kći, and then the clitic ga comes right after that.

That is why you get:

• moja kći ga radije čuva

rather than something more English-like such as:

• moja kći radije čuva ga ❌

That would sound unnatural in standard Croatian.

Radije means rather, preferably, or would rather.

It comes from rado:

• rado = gladly, willingly
• radije = more gladly

In natural English, ga radije čuva is often best translated as:

• she prefers to save it
• she would rather save it

So although the literal sense is she keeps/saves it more willingly, the natural meaning is she prefers to save it.

Because the sentence describes habitual behavior, not just something happening right now.

So the meaning is:

• My son spends pocket money on comics = that is what he generally does
• my daughter prefers to save it for books = that is her usual habit

Croatian uses the present tense for this just like English often does.

Also, both verbs are imperfective:

• trošiti = to be spending / to spend habitually
• čuvati = to keep / save habitually

That fits a general statement about repeated actions.

Yes, kći (daughter) is a somewhat irregular and old-looking feminine noun, so learners often notice it.

In this sentence it is just nominative singular:

• moja kći

You do not need to memorize all its forms immediately, but it is useful to know that it does not behave as simply as a regular noun like knjiga. That is one reason it may look a bit strange at first.

It can mean both, depending on context.

The basic meaning of čuvati is to keep, to guard, or to preserve. But with money, especially with za + purpose, it often means:

• to save
• to put aside

So here:

• ga radije čuva za knjige

is most naturally understood as:

• she prefers to save it for books

rather than just she keeps it for books.