Kiedy słychać grzmot, dzieci wracają do domu.

Here kiedy introduces a time clause, so it means when.

In a sentence like Kiedy słychać grzmot, dzieci wracają do domu, it often has a more general meaning, close to whenever in English:

• When / whenever thunder is heard, the children go back home.

So grammatically it is when, but in context it can describe a repeated situation.

Słychać means one can hear, it can be heard, or simply you hear in an impersonal sense.

It is used when Polish does not want to name a specific person who is hearing something. So:

• Słychać grzmot = Thunder can be heard / You can hear thunder

This is very natural in Polish. English often uses passive-like phrasing or an indefinite you, while Polish often uses słychać.

Both can be understood, but słychać is the more natural and idiomatic choice here.

Compare:

• Słychać grzmot = You can hear thunder / Thunder is audible
• Słyszy się grzmot = grammatically possible, but less natural in this context

So learners should think of słychać as a very common fixed way to say that a sound is heard.

Yes, grzmot is in the accusative case, because it is the thing being heard.

For the masculine noun grzmot:

• nominative: grzmot
• accusative: grzmot

These two forms happen to be the same for this noun, so there is no visible change.

That is why you see:

• słychać grzmot

even though grzmot is functioning as the object.

Grzmot usually refers to a clap/peal/rumble of thunder rather than the whole weather event.

Useful distinctions:

• grzmot = a thunderclap, a rumble of thunder
• burza = storm, thunderstorm
• piorun = lightning bolt

So this sentence is specifically about hearing thunder, not just about there being a storm.

Singular is completely natural here. It means that once a thunderclap is heard, the children go home.

Polish often uses singular in this kind of general statement:

• Kiedy słychać grzmot... = When thunder is heard...

You could use a plural form in another context, but singular is the most straightforward and idiomatic here.

Wracają is the imperfective present form of wracać.

It is used because the sentence describes a general, repeated situation:

• When thunder is heard, the children go back home.

This is not about one single future event, but about a habitual reaction.

Compare:

• dzieci wracają do domu = the children go back home / return home as a usual action
• dzieci wrócą do domu = the children will return home in one specific future situation

So wracają fits the general rule-like meaning.

After do, Polish requires the genitive case.

• dom = nominative
• domu = genitive

So:

• do domu = to home / to the house

This is the normal way to express movement toward home.

Compare:

• wracają do domu = they return home
• są w domu = they are at home

So do domu shows direction, while w domu shows location.

Because Kiedy słychać grzmot is a subordinate clause.

In Polish, when a sentence begins with a clause introduced by words like kiedy, gdy, jeśli, ponieważ, a comma normally separates that clause from the main clause.

So:

• Kiedy słychać grzmot, dzieci wracają do domu.

This is standard Polish punctuation.

Yes. Polish word order is more flexible than English word order.

For example, you could also say:

• Dzieci wracają do domu, kiedy słychać grzmot.

This still means basically the same thing.

The version with Kiedy... first puts the time condition in focus first, which is very natural. English can also do this: When thunder is heard, the children go home.

That is exactly the point of słychać: it is impersonal.

Polish does not need to say who hears it. The idea is simply that the sound is audible.

So the sentence does not mean that the children are necessarily the ones hearing it in a grammatically explicit way. It just means:

• When thunder can be heard, the children go home.

This kind of impersonal construction is very common in Polish.

Dzieci is plural here: children.

It is the plural form of dziecko = child.

So:

• dziecko wraca do domu = the child goes back home
• dzieci wracają do domu = the children go back home

You can also see it is plural because the verb is wracają, which is a plural form.

You could, but it sounds unnatural in English.

A more natural English translation would be:

• When thunder is heard, the children go home.
• When they hear thunder, the children go home.
• Whenever thunder is heard, the children go back home.

So a very literal translation is possible for analysis, but not always best for normal English.

The dictionary form is wracać.

Here is the relationship:

• wracać = imperfective infinitive, to return / to be returning / to go back
• wracają = they return / they are returning

The perfective partner is:

• wrócić = to return in the sense of completing the action once

This imperfective/perfective pairing is an important feature of Polish verbs.

Yes, gdy could also work:

• Gdy słychać grzmot, dzieci wracają do domu.

Both mean when. In many contexts they are interchangeable.

Very roughly:

• kiedy is extremely common and neutral
• gdy can sound a little more formal or literary in some contexts, though it is also common in normal speech

So kiedy is a very good default choice for learners.