Kot schował się pod łóżkiem, kiedy usłyszał grzmot.

Schował się means hid or went and hid.

The word się is very common in Polish. In this sentence, it is part of the verb schować się, which means to hide oneself / to hide.

So:

• schować = to put away, to store, to hide something
• schować się = to hide oneself

Compare:

• Schował książkę. = He put away / hid the book.
• Schował się. = He hid.

In English, we do not usually say hid himself, but in Polish this reflexive structure is normal.

This is a question of aspect, which is very important in Polish verbs.

• chować = imperfective
• schować = perfective

In this sentence, the cat did a completed single action: it hid when it heard thunder. Because the action is seen as complete, Polish uses the perfective verb:

• Kot schował się... = The cat hid...

If you used chował się, it would suggest something more ongoing, repeated, or descriptive, for example:

• Kot chował się pod łóżkiem za każdym razem, gdy słyszał grzmot.
  = The cat used to hide under the bed every time it heard thunder.

So schował się fits the one-time completed action.

This is the same aspect idea.

• słyszeć = to hear, to be hearing
• usłyszeć = to hear, in the sense of catching a sound or hearing something happen

Here the cat suddenly heard a thunderclap, so Polish uses the perfective form:

• kiedy usłyszał grzmot = when it heard a thunderclap

Compare:

• słyszał grzmot = he was hearing thunder / he could hear thunder
• usłyszał grzmot = he heard a thunderclap

So usłyszał marks a single completed event.

Because pod can take different cases depending on meaning.

With pod:

• pod + instrumental = location, under
• pod + accusative = movement to under

Here the cat is located under the bed after hiding, so Polish says:

• pod łóżkiem = under the bed

Compare:

• Kot jest pod łóżkiem. = The cat is under the bed.
• Kot wszedł pod łóżko. = The cat went under the bed.

So:

• pod łóżkiem = where?
• pod łóżko = to where?

Łóżkiem is the instrumental singular form of łóżko.

The basic noun is:

• łóżko = bed

After pod in a location meaning, Polish uses the instrumental, so:

• łóżko → łóżkiem

This is a very common pattern for neuter nouns ending in -o:

• okno → oknem
• krzesło → krzesłem
• łóżko → łóżkiem

So pod łóżkiem literally means something like under bed-INSTR.

The subject is still kot.

Polish often omits repeated subjects when they are clear from context. So in this sentence:

• Kot schował się pod łóżkiem, kiedy usłyszał grzmot.

the second verb usłyszał still refers to the cat.

In English, we usually repeat the subject or use a pronoun:

• The cat hid under the bed when it heard thunder.

In Polish, repeating kot would usually sound unnecessary here, because the verb form already helps show who is doing the action.

Yes. In the Polish past tense, verb forms show gender in the singular.

• schował
• usłyszał

Both are masculine singular past forms.

That matches kot, which is a masculine noun.

Compare:

• kot schował się, usłyszał = masculine
• kotka schowała się, usłyszała = feminine

So if the subject were kotka (female cat), the sentence would be:

• Kotka schowała się pod łóżkiem, kiedy usłyszała grzmot.

Grzmot means a thunderclap, peal of thunder, or a loud thunder sound.

So it is slightly more specific than the general English word thunder.

Useful comparison:

• burza = storm
• piorun = lightning bolt
• grzmot = thunderclap / rumble of thunder

So:

• usłyszał grzmot = it heard a thunderclap

In natural English, this is often translated simply as heard thunder.

Because Polish normally uses a comma before a subordinate clause introduced by words like:

• kiedy = when
• że = that
• bo = because
• jeśli = if

So:

• Kot schował się pod łóżkiem, kiedy usłyszał grzmot.

This comma is standard Polish punctuation.

English is different: English usually does not put a comma before when in this kind of sentence if the when-clause comes second.

Yes. In this sentence, gdy would be very natural and means almost the same thing.

• Kot schował się pod łóżkiem, kiedy usłyszał grzmot.
• Kot schował się pod łóżkiem, gdy usłyszał grzmot.

Both mean The cat hid under the bed when it heard a thunderclap.

Very roughly:

• kiedy is often the more neutral everyday word for when
• gdy can sound a little more written or literary in some contexts, though it is still very common

In this sentence, either works well.

Yes, Polish word order is more flexible than English, although some orders sound more natural in context than others.

The original sentence is very natural:

• Kot schował się pod łóżkiem, kiedy usłyszał grzmot.

You could also say:

• Kiedy usłyszał grzmot, kot schował się pod łóżkiem.

This puts more focus on the hearing of the thunderclap first.

You can also move some parts for emphasis, but not every version sounds equally neutral. Polish word order often reflects focus, emphasis, or style, not just grammar.

In this sentence, it describes where the cat ended up hiding: under the bed.

Polish often uses expressions like this with verbs of motion or change of state in ways that may feel slightly different from English. The important meaning is:

• the cat reacted to the thunderclap
• it hid
• the hiding place was under the bed

If you wanted to emphasize movement to a place under the bed, you might see a different structure, for example:

• Kot schował się pod łóżko — this is much less standard than verbs of motion like wszedł pod łóżko
• more naturally: Kot wszedł pod łóżko i tam się schował.
  = The cat went under the bed and hid there.

So in your sentence, pod łóżkiem is best understood as the location of the hiding place.

A rough English-friendly guide:

• łóżkiem ≈ WOO-shkyem
• grzmot ≈ GZH-mot

A few details:

• ł sounds like English w
• ó sounds like oo
• ż sounds like the s in measure
• rz here also sounds like ż
• ki before e is softened, so kie sounds roughly like kye

These are only approximations, but they can help at first.