Kad se boji grmljavine, djevojčica čvrsto stisne medvjedića.

Kad means when, but with present-tense verbs like here, it often has the sense of whenever.

So:

• Kad se boji grmljavine... = When/Whenever she is afraid of thunder...

In Croatian, kad is very common in everyday speech. You can also see kada, which means the same thing but sounds a bit more formal or full.

Because the verb is bojati se, which means to be afraid or to fear.

The se is part of the verb itself. It is not optional here.

• bojati se = to be afraid
• boji se = he/she is afraid

Many Croatian verbs are learned together with se, so it is best to memorize them as one unit.

Grmljavine is in the genitive singular.

That happens because bojati se normally takes the genitive:

• bojati se + genitive = to be afraid of something

So:

• grmljavina = thunder, thunderstorm noise, rumbling thunder
• grmljavine = of thunder

This is something English speakers often have to get used to: in Croatian, certain verbs require a specific case instead of a preposition like of.

The basic dictionary form is grmljavina.

Here is the change:

• grmljavina = nominative singular
• grmljavine = genitive singular

So in the sentence, the verb boji se forces grmljavina to become grmljavine.

Usually djevojčica means little girl or young girl.

It is related to:

• djevojka = girl, young woman
• djevojčica = little girl

So this sentence gives a childlike image, which fits well with medvjedića.

Croatian does not have articles like the or a/an.

So djevojčica can mean:

• a girl
• the girl
• the little girl

The exact meaning depends on context. English has to choose an article, but Croatian does not.

Čvrsto is an adverb, and here it means firmly, tightly, or hard.

It describes how she squeezes the teddy bear:

• čvrsto stisne = squeezes tightly / gives a firm squeeze

So čvrsto modifies the verb stisne.

This is a very useful aspect question.

• stisnuti is perfective
• stiskati / stiskivati or sometimes stiskati in use = imperfective
• stišće is a present form of an imperfective verb meaning is squeezing / squeezes repeatedly
• stisne is a present form of a perfective verb, focusing on a single completed squeeze

In this sentence, stisne suggests that when she gets scared, she gives the teddy bear a tight squeeze.

So the action is seen as a whole, almost like one complete reaction.

If you said čvrsto stišće medvjedića, that would feel more like:

• she is holding/squeezing it continuously

Medvjedića is accusative singular of medvjedić.

The important point is that medvjedić is a masculine animate noun, so in the singular its accusative form matches the genitive form.

• nominative: medvjedić
• accusative: medvjedića

Since he is the direct object of stisne, accusative is required.

Also, medvjedić literally means little bear, but in context it often means teddy bear.

It can mean either, depending on context.

In this sentence, because a little girl is hugging or squeezing it in fear, medvjedić is naturally understood as a teddy bear.

If you wanted to be extra explicit, you could say:

• plišani medvjedić = teddy bear / plush bear

But medvjedić alone is often enough when the context is clear.

Yes. Croatian word order is flexible, though the original sentence sounds natural and neutral.

Original:

• Kad se boji grmljavine, djevojčica čvrsto stisne medvjedića.

You could also say:

• Djevojčica čvrsto stisne medvjedića kad se boji grmljavine.

Both are grammatical. The original version puts the when-clause first, which is very common.

It most naturally sounds like a usual or typical reaction:

• When/Whenever she is afraid of thunder, the little girl tightly squeezes her teddy bear.

That habitual meaning comes from:

• kad = when/whenever
• present-tense style
• the general setup of the sentence

So it is less about one single moment and more about what she typically does.

Not exactly.

Grmljavina refers specifically to thunder or the sound of thunder. It is not the same as a whole storm.

Compare:

• grmljavina = thunder
• oluja = storm
• nevrijeme = bad weather / stormy weather

So boji se grmljavine means she is afraid of thunder, not necessarily of all storm weather in general.