Mój pies ciągle szczeka, kiedy słyszy dzwonek do drzwi.

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

The possessive adjective mój has to agree with the noun in gender, number, and case:

• mój pies = my dog
• moja książka = my book
• moje dziecko = my child

So here, mój matches pies.

Pies is nominative, and nominative is used for the subject of the sentence: the dog is the one doing the barking and hearing.

Compare:

• Mój pies szczeka. = My dog is barking / My dog barks.
• Widzę psa. = I see the dog.

In the second example, psa is in the accusative/genitive form, because the dog is the object, not the subject.

Ciągle means constantly, all the time, or keeps on.

In this sentence, it suggests repeated or ongoing barking whenever the dog hears the doorbell. A natural English sense is something like:

• My dog keeps barking whenever he hears the doorbell.

Similar words include:

• stale = constantly
• cały czas = all the time

In many everyday contexts, ciągle and stale can be quite close, though ciągle often feels very common and conversational.

Szczeka is the 3rd person singular present tense of szczekać = to bark.

It matches pies:

• ja szczekam = I bark
• ty szczekasz = you bark
• on/ona/ono szczeka = he/she/it barks

Since pies is singular, we use szczeka.

Because słyszeć means to hear, while słuchać means to listen (to).

The sentence is about the dog reacting when a sound reaches him, not about him intentionally listening.

• słyszy dzwonek = hears the doorbell
• słucha dzwonka / słucha czegoś = listens to the bell / listens to something

So kiedy słyszy dzwonek do drzwi is the correct choice for when he hears the doorbell.

Because this sentence describes a habitual/repeated action, not one single completed event.

• słyszy is imperfective: hearing in a general, repeated sense
• usłyszy is perfective: hears once, catches the sound, will hear

Here the meaning is basically:

• Whenever he hears the doorbell, he keeps barking.

That kind of repeated pattern usually takes the imperfective in Polish.

Polish often omits subject pronouns when they are clear from the verb form or context.

In this sentence, the subject is already mój pies, so there is no need to repeat on:

• Mój pies ciągle szczeka, kiedy słyszy dzwonek do drzwi.

It is understood that słyszy refers to the dog.

You could say kiedy on słyszy..., but it would usually sound unnecessary unless you wanted special emphasis.

Yes, gdy would also work.

Both kiedy and gdy can mean when in a sentence like this:

• Mój pies ciągle szczeka, kiedy słyszy dzwonek do drzwi.
• Mój pies ciągle szczeka, gdy słyszy dzwonek do drzwi.

Kiedy is very common and neutral in everyday speech.
Gdy can sound a little more literary or formal in some contexts, but it is also perfectly normal.

Because kiedy słyszy dzwonek do drzwi is a subordinate clause.

Polish punctuation normally puts a comma before subordinate conjunctions like:

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

So the comma in:

• Mój pies ciągle szczeka, kiedy słyszy dzwonek do drzwi.

is standard and correct.

Dzwonek do drzwi is the standard Polish expression for doorbell.

Literally, it is something like:

• dzwonek = bell
• do drzwi = for the door / to the door

So the whole phrase means the bell for the door, which is how Polish expresses doorbell.

It is best learned as a set phrase:

• słyszeć dzwonek do drzwi = to hear the doorbell
• zadzwonił dzwonek do drzwi = the doorbell rang

The preposition do requires the genitive case.

The noun drzwi is a special noun because it exists only in the plural in Polish. Its nominative and genitive plural forms are both drzwi, so the form does not change here:

• drzwi = door / doors
• do drzwi = to the door / of the door

So even though there is a case change required by do, the word still looks the same.

Because Polish has no articles like English a and the.

So:

• mój pies can mean my dog
• dzwonek do drzwi can mean the doorbell or a doorbell, depending on context

The exact meaning is understood from context rather than from articles.

Yes. Polish word order is fairly flexible, although some orders sound more natural than others depending on emphasis.

The given sentence is very natural:

• Mój pies ciągle szczeka, kiedy słyszy dzwonek do drzwi.

You could also say:

• Kiedy mój pies słyszy dzwonek do drzwi, ciągle szczeka.

That puts more focus on the when-clause first.

Polish word order often changes emphasis rather than basic meaning.

Not necessarily. In everyday speech, ciągle is often used a bit loosely.

Here it most naturally means that the dog keeps barking or barks repeatedly in that situation. It does not have to mean one endless uninterrupted bark.

So the sense is more like:

• Whenever he hears the doorbell, he just keeps barking.

rather than a mathematically exact 100% nonstop meaning.

Yes, but the meaning changes slightly.

• ciągle szczeka = keeps barking / barks continuously or repeatedly
• zawsze szczeka = always barks

Zawsze emphasizes that it happens every time.
Ciągle emphasizes the ongoing/repeated nature of the barking.

So both are possible, but they are not exactly the same.