Nasz kot ucieka do sypialni, kiedy słyszy grzmot.

Because nasz must agree with the noun it describes.

• kot is masculine singular
• so the correct form is nasz

Compare:

• nasz kot = our cat
• nasza sypialnia = our bedroom
• nasze okno = our window

So the ending changes depending on the gender of the noun.

Here kot is the subject of the sentence, so it is in the nominative case.

• kot = nominative singular
• kota can be genitive or accusative, depending on context

In this sentence, the cat is the one doing the action:

• Nasz kot ucieka... = Our cat runs away...

So nominative kot is correct.

Because the preposition do always requires the genitive case.

The noun is:

• sypialnia = bedroom

Its genitive singular form is:

• sypialni

So:

• do sypialni = to the bedroom / into the bedroom

This is a very common pattern in Polish:

• do domu = to the house
• do szkoły = to school
• do kuchni = to the kitchen

It is genitive singular.

Why?

• dictionary form: sypialnia
• after do, Polish uses the genitive
• so sypialnia changes to sypialni

This is one of the first case patterns learners meet, because do + genitive is extremely common.

Ucieka comes from uciekać, which is an imperfective verb.

Here it means something like:

• runs away
• flees
• escapes

In this sentence, it suggests a repeated or typical reaction:

• when the cat hears thunder, it runs off to the bedroom

Because it is imperfective, it does not focus on a single completed escape. It describes the action as a process or habitual event.

Because Polish, like English, often uses the present tense for habitual actions.

So:

• Nasz kot ucieka do sypialni, kiedy słyszy grzmot.

means something like:

• Our cat runs to the bedroom when it hears thunder.
• Our cat tends to run to the bedroom when it hears thunder.

This is a normal way to describe repeated behavior.

Because kiedy introduces a subordinate clause.

Main clause:

• Nasz kot ucieka do sypialni

Subordinate clause:

• kiedy słyszy grzmot

In Polish, subordinate clauses are normally separated by a comma, so the comma is required here.

Here kiedy means when.

It connects the two parts of the sentence:

• the cat runs to the bedroom
• when it hears thunder

So kiedy słyszy grzmot = when it hears thunder

Depending on context, kiedy can also mean whenever, especially in sentences describing habitual actions like this one.

Because słyszeć usually takes a direct object in the accusative case.

The noun grzmot is a masculine inanimate noun. For masculine inanimate nouns in the singular:

• nominative = grzmot
• accusative = grzmot

So the form does not change.

That is why you see:

• słyszy grzmot = hears thunder / hears a thunderclap

If the noun were masculine animate, you would often see a different accusative form.

Grzmot means a thunderclap, peal of thunder, or crash of thunder.

It refers to the sound, not the lightning itself.

Useful related words:

• grzmot = thunderclap / crash of thunder
• piorun = lightning / thunderbolt
• burza = storm / thunderstorm

So in this sentence, the cat reacts to the sound of thunder.

Yes. Polish word order is fairly flexible, although some orders sound more natural in neutral contexts.

For example, you could also say:

• Kiedy słyszy grzmot, nasz kot ucieka do sypialni.

This means the same thing.

The original version is neutral and natural:

• Nasz kot ucieka do sypialni, kiedy słyszy grzmot.

Changing word order often changes emphasis, not the basic meaning.

Yes, very often.

In English, we may distinguish:

• to the bedroom
• into the bedroom

Polish commonly uses do + genitive for movement toward and into an enclosed space, so do sypialni can cover that idea naturally.

In this sentence, it strongly suggests the cat goes into the bedroom.

In this sentence, słyszy refers to nasz kot.

Polish often leaves pronouns like he, she, or it unstated, because the verb form and context usually make the subject clear.

So the structure is understood as:

• Nasz kot ucieka do sypialni, kiedy [nasz kot] słyszy grzmot.

English would normally repeat the subject with it, but Polish does not need to.

Słyszy is the 3rd person singular present form of słyszeć.

So:

• ja słyszę = I hear
• ty słyszysz = you hear
• on/ona/ono słyszy = he/she/it hears

Here the subject is kot, so słyszy means hears.

Grammatically, kot is a masculine noun, so Polish grammar treats it as masculine.

That affects agreement:

• nasz kot
• słyszy
• if needed, later adjectives or past tense forms would also match masculine grammar

But in English, cat is often translated as it unless the animal’s sex matters. So the meaning can still be rendered naturally in English as our cat or it depending on context.