Muszę zejść na dół, bo dzwoni dzwonek do drzwi.

In Polish, the subject pronoun is often omitted because the verb ending already shows who is doing the action.

• muszę = I must / I have to
• the ending -ę shows 1st person singular

So Muszę zejść na dół is a complete sentence by itself. You would add ja only for emphasis or contrast, for example:

• Ja muszę zejść na dół, a ty zostań tutaj.
  = I have to go downstairs, and you stay here.

Zejść is the infinitive used after muszę.

• muszę zejść = I have to go down / go downstairs
• zejść is a perfective verb

That means it presents the action as a single completed movement: the speaker needs to go down now and actually get there.

A common imperfective partner is schodzić. Compare:

• Muszę zejść na dół.
  = I need to go downstairs now.
• Muszę schodzić na dół codziennie.
  = I have to go downstairs every day.

So here zejść fits the one-time, complete action.

It may look redundant from an English point of view, but in Polish zejść na dół is a very normal, everyday phrase.

• zejść already contains the idea of descending
• na dół makes the direction or destination extra clear: downstairs / down below

So yes, there is some overlap in meaning, but the whole phrase is completely natural.

You can sometimes say just Muszę zejść, but that depends on context. Without na dół, it can feel less specific.

These are easy to mix up:

• na dół = downstairs / down as a destination or direction of movement
  
  • Idę na dół. = I’m going downstairs.
• na dole = downstairs / below as a location
  
  • Jestem na dole. = I’m downstairs.
• w dół = downward / downwards, often more about direction than destination
  
  • Patrzę w dół. = I’m looking down.
  • Schody prowadzą w dół. = The stairs lead down.

In your sentence, the speaker is moving to a lower place, so na dół is the natural choice.

Because na changes case depending on meaning.

With movement toward a place, na takes the accusative:

• na dół = downward / downstairs

With location in a place, na takes the locative:

• na dole = downstairs

So this is the same kind of contrast as:

• Idę na górę. = I’m going upstairs.
• Jestem na górze. = I’m upstairs.

Bo means because.

• Muszę zejść na dół, bo dzwoni dzwonek do drzwi.
  = I have to go downstairs, because the doorbell is ringing.

In standard Polish spelling, you normally put a comma before bo when it introduces another clause. So the comma here is required.

Because Polish can express this idea by saying that the doorbell itself is ringing.

• dzwoni = is ringing
• dzwonek do drzwi = doorbell

So dzwoni dzwonek do drzwi is a natural way to say that the bell is making a sound.

Another common Polish sentence is:

• Ktoś dzwoni do drzwi.
  Literally: Someone is ringing at the door.
  Natural English meaning: Someone is ringing the doorbell.

So Polish can describe either:

1. the bell making noise, or
2. the person pressing it.

Literally, it means a bell for the door.

• dzwonek = bell, little bell, doorbell
• do drzwi = for the door

This is a very common Polish pattern:

• klucz do drzwi = key for the door
• bilet do kina = ticket for the cinema
• pilot do telewizora = remote control for the TV

So dzwonek do drzwi is simply the normal way to say doorbell.

Two separate things are happening here:

1. Why do drzwi?
Because do requires the genitive case.

2. Why does drzwi look plural?
Because drzwi is a plural-only noun in Polish. Grammatically, it behaves like a plural word even though in English we usually say door as singular.

For example:

• Te drzwi są otwarte. = The door is open.
  Literally, Polish uses plural grammar.

So in dzwonek do drzwi, the form drzwi is correct and normal.

Yes, absolutely.

• Muszę iść na dół = I have to go downstairs
• Muszę zejść na dół = I have to go downstairs

Both are natural, but there is a small difference:

• iść is more general: to go
• zejść is more specific: to go down / descend

So zejść highlights the downward movement more clearly.

Yes. Polish word order is flexible, and both versions are possible.

• Dzwonek do drzwi dzwoni.
• Dzwoni dzwonek do drzwi.

The difference is mostly about emphasis:

• Dzwonek do drzwi dzwoni sounds a bit more neutral and descriptive
• Dzwoni dzwonek do drzwi gives a little more focus to the event itself, almost like The doorbell’s ringing!

So the original word order is perfectly natural.