Kiedy dzwoni dzwonek do drzwi, pies biegnie do przedpokoju.

Kiedy means when here and introduces a subordinate clause:

Kiedy dzwoni dzwonek do drzwi = When the doorbell rings

So the whole sentence has two parts:

• Kiedy dzwoni dzwonek do drzwi = the time clause
• pies biegnie do przedpokoju = the main clause

Polish can also reverse the order:

Pies biegnie do przedpokoju, kiedy dzwoni dzwonek do drzwi.

That means the same thing, though the original order is very natural.

Because the clause beginning with kiedy is a subordinate clause, and in Polish such clauses are normally separated by a comma from the main clause.

So:

Kiedy dzwoni dzwonek do drzwi, pies biegnie do przedpokoju.

This is standard Polish punctuation.

Both are possible, because Polish word order is more flexible than English word order.

• dzwoni dzwonek sounds very natural here
• dzwonek dzwoni is also understandable and grammatical

The version with the verb first often feels a bit more focused on the action happening. After kiedy, Polish often naturally allows this kind of order.

So the sentence is not using a strange grammar rule here; it is just using normal Polish flexibility.

Because they come from the same root:

• dzwonek = bell, little bell, doorbell
• dzwonić = to ring

So dzwoni dzwonek literally has the same kind of relationship as the bell rings in English. It may look repetitive to an English speaker, but in Polish it is completely normal.

It means doorbell.

Literally, it is something like a bell for the door. In Polish, do is often used in noun phrases to show purpose or connection, not only motion.

Compare:

• klucz do drzwi = key to the door
• szczotka do włosów = hairbrush
• dzwonek do drzwi = doorbell

So here do does not mean someone is moving anywhere. It is just part of the expression naming the object.

Drzwi is one of those Polish nouns that is grammatically plural even when it refers to a single door.

This is called a plural-only noun.

So Polish says:

• te drzwi = this door / these doors, depending on context
• do drzwi = to the door

After do, you need the genitive case, but drzwi stays drzwi here. That is why the form does not visibly change.

Because pies is the subject of the sentence, so it is in the nominative case.

• pies = the dog, as the subject
• psa would be accusative or genitive, depending on context

Here the dog is doing the action:

pies biegnie = the dog runs

So nominative pies is the correct form.

Because the preposition do requires the genitive case.

The basic dictionary form is:

• przedpokój = hallway, entry hall

After do, it becomes:

• do przedpokoju = to the hallway / into the hallway

So:

• nominative: przedpokój
• genitive: przedpokoju

This is a very common pattern in Polish.

Because do shows movement toward a destination, while w usually shows location.

• biegnie do przedpokoju = the dog runs to / into the hallway
• biegnie w przedpokoju = the dog is running in the hallway

So the original sentence is about where the dog goes, not where the dog already is.

Because Polish often uses the present tense for habitual or general repeated actions.

This sentence means something like:

Whenever the doorbell rings, the dog runs to the hallway.

So:

• dzwoni = rings
• biegnie = runs

Both are present-tense imperfective forms, which fit a repeated, usual action very well.

It can mean either, depending on context.

Polish present tense does not map perfectly onto English present forms. So biegnie may mean:

• runs
• is running

In this sentence, because the whole sentence describes a usual reaction, the best English translation is normally runs.

Then Polish would usually use perfective verbs:

Kiedy zadzwoni dzwonek do drzwi, pies pobiegnie do przedpokoju.

That means:

When the doorbell rings, the dog will run to the hallway.

Compare the two versions:

• Kiedy dzwoni dzwonek do drzwi, pies biegnie do przedpokoju.
  = habitual, repeated situation

• Kiedy zadzwoni dzwonek do drzwi, pies pobiegnie do przedpokoju.
  = one future event

This is an important Polish aspect difference.

Yes. In this sentence, gdy would work too:

Gdy dzwoni dzwonek do drzwi, pies biegnie do przedpokoju.

That also means when the doorbell rings, the dog runs to the hallway.

Very roughly:

• kiedy is extremely common and neutral
• gdy is also common, sometimes felt as slightly more formal or written, depending on context

But in a sentence like this, both are perfectly normal.

Because Polish has no articles like a or the.

So:

• pies can mean a dog or the dog
• dzwonek can mean a bell or the bell

Context tells you which meaning is intended. In this sentence, English naturally uses the because both the dog and the doorbell are understood as specific in the situation.