Kad djeca zaspu, baka mi pokaže staru sliku na kojoj zmaj čuva šumu, a iznad njega leti sova.

Kad and kada both mean when. In this sentence, kad is just the shorter, very common form. It is especially common in everyday speech and normal writing. Kada can sound a bit more formal or fuller, but both are correct here.

Djeca means children and it takes plural agreement:

• djeca zaspu = the children fall asleep

So even though it may not look like a typical plural ending to an English speaker, it behaves as plural in Croatian.

Some related forms are:

• djeca = nominative plural
• djece = genitive plural
• djeci = dative/locative plural

Because zaspu comes from zaspati, which means to fall asleep, not just to sleep.

• zaspati / zaspu = fall asleep, reach the sleeping state
• spavati / spavaju = sleep, be sleeping

So:

• Kad djeca zaspu = When the children fall asleep
• Kad djeca spavaju would mean something more like When the children are sleeping

The sentence wants the moment the children drift off, so zaspu is the right choice.

Pokaže is from the perfective verb pokazati. It presents the action as a complete event: grandma shows me the picture as one whole action.

• pokazati → pokaže = show, in a complete/single-event sense
• pokazivati → pokazuje = be showing / show repeatedly / show in an ongoing sense

Here the sentence has a kind of narrative sequence:

• the children fall asleep
• grandma shows me the picture

So pokaže fits that whole completed event feeling very well.

Mi means to me. It is the dative clitic form of ja.

So:

• baka mi pokaže = grandma shows me

Why is it there? Because mi is an unstressed clitic, and Croatian clitics usually appear near the beginning of the clause, often in second position.

That is why baka mi pokaže sounds natural, while baka pokaže mi does not.

Because sliku is the direct object of pokaže, so it must be in the accusative.

The noun is feminine singular:

• nominative: stara slika
• accusative: staru sliku

Both the adjective and the noun change:

• stara → staru
• slika → sliku

So baka mi pokaže staru sliku means grandma shows me an old picture.

Because the relative pronoun takes the case required inside its own clause, not the case of the noun before it.

The noun before it is:

• sliku = accusative, because it is the object of pokaže

But inside the relative clause, the pronoun is used with na:

• na kojoj = on which

The preposition na here requires the locative, so we get:

• koja → kojoj

Also, it agrees with slika, which is feminine singular.

So:

• sliku na kojoj zmaj čuva šumu = the picture on which a dragon guards the forest

Croatian usually treats the content of a picture as being on its surface, so na is natural.

That is why you get:

• na slici = in/on the picture
• na kojoj = on which

Even though English often says in the picture, Croatian normally says na slici.

Because šuma is the direct object of čuva.

• zmaj čuva šumu = the dragon guards the forest

Since šuma is feminine singular, its accusative form is šumu.

• nominative: šuma
• accusative: šumu

Iznad takes the genitive, so the pronoun after it must be in a genitive form.

Here njega refers to zmaj:

• zmaj = masculine singular
• iznad njega = above him / above it

So the structure is:

• iznad
  • genitive

That is why it is njega, not on.

It also shows clearly that the owl is flying above the dragon, not above the picture. If it referred to slika, you would expect a feminine form instead.

Croatian word order is more flexible than English word order.

Both are possible:

• sova leti
• iznad njega leti sova

In this sentence, iznad njega comes first to set the scene: above him. Then leti sova follows. This word order sounds natural and slightly more descriptive or literary.

English usually needs a more fixed order, but Croatian can move parts around for emphasis, rhythm, or style.

They mark the different parts of the sentence:

• Kad djeca zaspu,
  The comma closes the introductory time clause: When the children fall asleep, ...

• ..., a iznad njega leti sova.
  The comma before a separates two coordinated actions inside the picture:

  • zmaj čuva šumu
  • a iznad njega leti sova

Notice there is no comma before na kojoj, because that relative clause directly identifies the picture. It is a defining part of staru sliku.

Because Croatian normally has no articles. English says:

• the children
• an old picture
• the forest
• an owl

Croatian simply uses the noun itself:

• djeca
• stara slika
• šuma
• sova

Whether something is a, an, or the is usually understood from context.