Mačka se sakrila ispod kreveta kad je počela grmljavina.

Se is the reflexive clitic. With the verb sakriti se, it means to hide oneself / to hide.

So:

• sakriti = to hide something
• sakriti se = to hide oneself, to hide

In this sentence, the cat is doing the action to itself, so Croatian uses the reflexive form: se sakrila.

Because mačka is a grammatically feminine noun, and in the Croatian past tense, the l-participle agrees with the subject in gender and number.

So:

• masculine singular: sakrio
• feminine singular: sakrila
• neuter singular: sakrilo

Since the subject is mačka, we get sakrila.

A useful note for English speakers: even if you mean a male cat in real life, the noun mačka is still grammatically feminine. If you specifically want tomcat, Croatian usually uses mačak, which is masculine.

It is the past tense in Croatian, often called the perfect.

It is formed with:

• a present-tense form of biti (to be)
• plus the l-participle

In full, this sentence could be:

• Mačka se je sakrila...

But in standard Croatian, je usually disappears when it would come right after se, so the normal form is:

• Mačka se sakrila...

This is very common with short clitic words like se, je, ga, mu, and so on.

Because of Croatian clitic behavior.

In the past tense, you often expect je with a third-person singular subject:

• počela je
• došao je
• vidjela je

But when se is present, Croatian normally does not say se je in standard usage. So:

• expected structure: se je sakrila
• actual standard form: se sakrila

That is why you see Mačka se sakrila, not Mačka se je sakrila.

After the preposition ispod (under / below), Croatian uses the genitive case.

So:

• dictionary form: krevet = bed
• genitive singular: kreveta

That is why the phrase is:

• ispod kreveta = under the bed

This is something learners need to memorize with prepositions: many Croatian prepositions require a specific case.

Yes, ispod is used with the genitive.

Examples:

• ispod stola = under the table
• ispod mosta = under the bridge
• ispod prozora = below the window

So in your sentence, kreveta is genitive because ispod requires it.

Yes. Kad and kada both mean when.

• kad is shorter and very common in everyday speech
• kada is a bit fuller and can sound slightly more formal or careful, depending on context

So both are correct here:

• kad je počela grmljavina
• kada je počela grmljavina

This is also the Croatian past tense.

The subject of this part is grmljavina (thunder / thunderstorm / rumbling thunder, depending on context), and grmljavina is feminine. So the participle must also be feminine:

• počeo = masculine
• počela = feminine
• počelo = neuter

And Croatian normally uses the auxiliary je here:

• grmljavina je počela = the thunder started

In your sentence, because of word order after kad, you get:

• kad je počela grmljavina

Yes. That is grammatically possible.

Croatian word order is more flexible than English word order because case endings and verb forms carry a lot of grammatical information.

So these are all possible:

• kad je počela grmljavina
• kad je grmljavina počela

They are very similar in meaning, but the emphasis can shift slightly.
The version in your sentence sounds natural and neutral.

Because Croatian often places information in a flexible way depending on rhythm, emphasis, and style.

In kad je počela grmljavina, the verb phrase comes first and the subject comes after it. This is completely normal in Croatian, especially in narration.

English usually prefers:

• when the thunder started

Croatian can comfortably use:

• kad je počela grmljavina

without sounding strange.

This is a very important aspect pair.

• sakriti se = perfective
  It focuses on the completed action: to hide, to get hidden
• skrivati se = imperfective
  It focuses on duration, repetition, or process: to be hiding, to hide repeatedly

In your sentence, the cat performed a single completed action, so sakrila se is the natural choice.

Compare:

• Mačka se sakrila ispod kreveta.
  The cat hid under the bed.
• Mačka se skrivala ispod kreveta.
  The cat was hiding under the bed.

Yes, quite a lot, although some versions sound more natural than others.

For example:

• Mačka se sakrila ispod kreveta kad je počela grmljavina.
• Kad je počela grmljavina, mačka se sakrila ispod kreveta.
• Ispod kreveta se mačka sakrila kad je počela grmljavina.

The most neutral and natural alternatives are usually the first two.
Croatian uses word order partly for focus and emphasis, not only for grammar.

Because Croatian has no articles.

English says:

• the cat
• the bed
• the thunder

Croatian simply says:

• mačka
• krevet / kreveta
• grmljavina

Whether something is a or the is understood from context rather than from an article.

Not always exactly. Grmljavina refers to the sound or occurrence of thunder, and in context it can feel like thunder, thundering, or even a thunderstorm beginning.

So in this sentence, depending on the translation style, it might be understood as:

• when the thunder started
• when the thunderstorm began
• when it started thundering

The Croatian sentence itself is natural; the best English translation depends on the context.

Yes. Croatian allows that.

For example:

• Mačka se sakrila ispod kreveta...
• Mačka se ispod kreveta sakrila...
• Ispod kreveta se mačka sakrila...

But not all versions sound equally neutral. The original sentence is the most straightforward and natural for most contexts.

A good learner tip: understand that Croatian word order is flexible, but when speaking, start with the more neutral pattern you see in sentences like this one.