Nie idź przez środek ulicy.

Idź is the imperative form of iść.

• iść = to go / to walk in one direction
• idź! = go!
• nie idź! = don’t go!

In this sentence, idź is addressed to one person in an informal way.

Related forms:

• idźcie = imperative for more than one person
• formal singular commands are usually expressed differently, for example:
  • Proszę nie iść przez środek ulicy.
  • Niech pan/pani nie idzie przez środek ulicy.

Because iść and chodzić are not the same.

• iść = to go/walk in one specific direction, on one occasion
• chodzić = to go/walk habitually, repeatedly, or in no fixed direction

This sentence warns someone about a specific movement, so iść → idź is the natural choice.

Compare:

• Nie idź przez środek ulicy. = don’t go through the middle of the street
• Nie chodź po ulicy. = don’t walk around in the street / don’t be going about in the street

Also, chodź! very often means come on! / come!, so it would not fit well here.

In Polish, nie is normally written separately from verbs, including imperatives.

So:

• idź = go
• nie idź = don’t go

This is standard Polish spelling and grammar.

Here przez means something like through, across, or via.

It is used for movement that passes through an area or across a space. In this sentence, it tells you the route: the person should not go through the middle of the street.

A very important grammar point: przez takes the accusative case.

Because środek means middle or center, and Polish often builds phrases like this as:

• środek czegoś = the middle of something

So:

• środek ulicy = the middle of the street

The noun after środek goes into the genitive case.

That is why:

• dictionary form: ulica
• here: ulicy

Because środek is a masculine inanimate noun, and for many such nouns, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: środek
• accusative: środek

That is why you get:

• przez środek

Then ulicy stays in the genitive because it depends on środek:

• środek ulicy = middle of the street

So the structure is:

• przez
  • środek (accusative)
• ulicy (genitive, because of środek)

Because ulica is not the noun directly governed by przez here.

The preposition przez governs the whole phrase środek ulicy. The head noun of that phrase is środek, not ulica.

So:

• przez środek = through the middle
• środek ulicy = the middle of the street

If you said przez ulicę, that would mean across the street or through the street, which is a different structure and a different meaning.

Yes, and the meaning changes slightly.

• Nie idź przez środek ulicy.
  • Don’t go through/across the middle of the street.
• Nie idź na środek ulicy.
  • Don’t go to the middle of the street.
• Nie idź środkiem ulicy.
  • Don’t walk along the middle of the street.

So these are related, but not identical.

A native speaker might choose one or another depending on whether the focus is:

• the route through the middle,
• the destination in the middle,
• or moving along the middle.

The neutral, normal order is:

• Nie idź przez środek ulicy.

Polish word order is more flexible than English, so other orders are possible, but they sound more marked or emphatic.

For example:

• Przez środek ulicy nie idź.

This is possible, but it strongly emphasizes przez środek ulicy. A learner should usually stick with the standard order unless there is a special reason to emphasize something.

The hardest parts for many English speakers are idź, przez, and środek.

A rough guide:

• nie ≈ nyeh
• idź = one syllable; the final dź is a very soft sound, somewhat like a softened j
• przez begins with a Polish prz cluster, which is harder than it looks for English speakers
• środek begins with śr, where ś is a soft sh sound
• ulicy has c pronounced like ts

Stress:

• śROdek
• uLIcy

If IPA helps, one careful pronunciation is approximately:

• Nie idź przez środek ulicy.
• /ɲɛ idʑ pʂɛs ɕrɔdɛk ulit͡sɨ/

You do not need perfect IPA to learn it, but it helps to know that the Polish consonants here are much softer than English spelling might suggest.

Because Polish does not have articles like a and the.

So ulica can mean:

• a street
• the street

The exact meaning comes from context.

That is why Polish can simply say:

• środek ulicy

without needing a separate word for the.