Muszę zejść na dół po wodę.

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 tells you it is 1st person singular: I

So Ja muszę zejść na dół po wodę is possible, but ja is usually unnecessary unless you want emphasis, contrast, or clarity.

Muszę is the 1st person singular form of musieć, meaning to have to / must.

So:

• muszę = I have to / I must
• musisz = you have to
• musi = he/she/it has to

In everyday Polish, muszę often sounds like English I have to, not always as strong as formal English I must.

After musieć (to have to), Polish normally uses the infinitive of the second verb, just like English often does after have to.

• Muszę zejść = I have to go down
• Muszę iść = I have to go
• Muszę pracować = I have to work

So muszę is the conjugated verb, and zejść stays in the infinitive.

This is about the kind of movement being described.

• iść = to go / to be going in a general sense
• zejść = to go down / come down once, as a completed action
• schodzić = the imperfective partner of zejść, meaning to be going down, to go down repeatedly, or focusing on the process rather than completion

In Muszę zejść na dół po wodę, zejść is natural because it suggests a single, purposeful trip downward.

Compare:

• Muszę zejść na dół = I need to go downstairs
• Schodzę na dół = I’m going downstairs / I go downstairs
• Muszę schodzić na dół codziennie = I have to go downstairs every day

Na dół means down, downstairs, or to the lower floor/level.

It is made of:

• na = a preposition
• dół = bottom / lower part

Together, na dół is a fixed expression used with motion downward.

Examples:

• Idę na dół = I’m going downstairs
• Zejdź na dół = Go downstairs
• Rzuć to na dół = Throw it down

It is two words because it is a prepositional phrase, not a single adverb.

This is the common Polish distinction between motion and location.

• na dół = down / downstairs → movement toward a place
• na dole = downstairs / at the bottom → being in that place

So:

• Muszę zejść na dół = I have to go downstairs
• Jestem na dole = I am downstairs

The case changes too:

• na dół → accusative, because of movement
• na dole → locative, because of location

Po wodę means for water in the sense of to fetch/get water.

In Polish, iść / jechać / zejść / wrócić po coś often means going somewhere in order to get something.

So:

• iść po chleb = to go get bread
• pojechać po dzieci = to go pick up the children
• zejść po wodę = to go downstairs for water / to get water

This use of po is very common and important.

Because the preposition po in this meaning takes the accusative case.

The basic form is:

• woda = water (nominative)

But after po meaning to get/fetch, it changes to accusative:

• po wodę = for water

Since woda is a feminine noun, the accusative singular changes -a to -ę:

• kawa → po kawę
• herbata → po herbatę
• woda → po wodę

Not always. In this sentence, po wodę means to get water / to fetch water, not just for water in every possible English sense.

For example:

• Idę po wodę = I’m going to get water
• Skoczę po kawę = I’ll quickly go get coffee

So po + accusative often implies a purpose: you go somewhere and come back with the thing.

It is more specific than just a general English for.

No. It often means downstairs, but it can also mean down/downward more generally, depending on context.

Examples:

• Idę na dół = I’m going downstairs
• Spadł na dół = He fell down
• Zjedź na dół windą = Go down by elevator

So in this sentence, most likely it means downstairs, but the phrase itself is broader.

Polish word order is more flexible than English, but this version is the most neutral and natural.

• Muszę zejść na dół po wodę = neutral
• Po wodę muszę zejść na dół = emphasizes for water
• Na dół muszę zejść po wodę = possible, but more marked or context-driven

Because Polish uses case endings, word order often changes for emphasis rather than basic grammar.

By itself, zejść only means to go down / come down. It does not automatically include returning.

The idea of getting water is expressed by po wodę, which implies the purpose of the trip, but the sentence itself only literally says that the speaker has to go down for water.

If you wanted to make the return explicit, you would need more words.

Yes, it is very natural.

A Polish speaker would easily understand it as something like:

• I have to go downstairs for water
• I need to go down and get some water

It sounds like ordinary spoken Polish and uses very common structures:

• muszę + infinitive
• na dół for downward movement
• po + accusative for going to fetch something