Learn Hardcore Croatian: Grad je manje zagađen kad ljudi prestanu pušiti cigarete na ulici. - The city is less polluted when people stop smoking cigarettes in the street.

Je is the present tense of biti (to be). Croatian normally needs this verb in the present tense to link the subject (grad) with a description (manje zagađen).

Grad je manje zagađen. = The city is less polluted.
You cannot omit je in standard Croatian: ✗ Grad manje zagađen is ungrammatical.

You can move je a bit for emphasis (because it's a clitic), e.g. Grad je manje zagađen / Grad je zagađen manje, but it must be present somewhere in the clause.

What exactly is happening in manje zagađen? Why two words, and why this form?

Manje zagađen is made of:

manje – the comparative form of malo (little), used here as an adverb meaning less.
zagađen – a past passive participle from zagaditi (to pollute), used as an adjective meaning polluted.

Together: manje zagađen = less polluted.

Agreement details:

grad is masculine singular nominative.
zagađen is also masculine singular nominative, so it agrees with grad.
manje does not change for gender/number here; it just modifies zagađen.

Examples with agreement changes:

Grad je manje zagađen. – The city (m.sg.) is less polluted.
Rijeka je manje zagađena. – The river (f.sg.) is less polluted.
Sela su manje zagađena. – The villages (n.pl.) are less polluted.

Why is it kad, and what’s the difference between kad and kada?

Kad means when in a temporal sense. Here it introduces a time clause: when people stop smoking…

Kad ljudi prestanu… = When people stop…

Kad and kada are essentially the same word:

kad – shorter, more colloquial and very common in speech and informal writing.
kada – slightly more formal or emphatic, more common in careful or official writing.

You can freely replace it here:

Grad je manje zagađen kada ljudi prestanu pušiti cigarete na ulici.

In English we say “when people stop …”, which is future-related. Why is Croatian using the present form prestanu instead of a future tense?

In Croatian (and other Slavic languages), after time conjunctions like kad / kada, čim (as soon as), dok (while), you usually use the present tense, even when the meaning is future.

So:

Grad će biti manje zagađen kad ljudi prestanu pušiti…
Literally: The city will be less polluted when people stop smoking…
– prestanu is present form, but refers to a future event.

This is normal and correct. You don’t say:

✗ … kad će ljudi prestati pušiti… (ungrammatical)

Think of it as: after kad, Croatian switches to a “special” use of the present that covers future time in this context.

Why prestanu and not prestaju? What’s the difference?

Two different verbs are involved:

prestati (perfective) → present forms: prestanem, prestaneš, prestanu…
prestajati (imperfective) → present forms: prestajem, prestaješ, prestaju…

In your sentence:

prestanu = 3rd person plural present of prestati (perfective).

Nuance:

prestanu (perfective) focuses on the act of reaching the point of stopping (the change).
prestaju (imperfective) would focus on an ongoing or repeated process of stopping, and would sound off in this type of generic rule.

For general “once they stop…” / “whenever they stop…”, Croatian normally uses the perfective here:

Grad je manje zagađen kad ljudi prestanu pušiti… ✅
Grad je manje zagađen kad ljudi prestaju pušiti… – sounds unusual/wrong in this meaning.

Could we say kad ljudi ne puše cigarete na ulici instead of kad ljudi prestanu pušiti cigarete na ulici?

Yes, but the nuance changes slightly.

kad ljudi prestanu pušiti cigarete na ulici
– focuses on the moment (or fact) that people stop doing it, i.e. there used to be smoking, then it stops.
kad ljudi ne puše cigarete na ulici
– simply describes times / situations when they are not smoking on the street, without stressing a change from “smoking” to “not smoking”.

Both can describe roughly the same real-world situation, but prestanu emphasizes the ending of the habit.

What case is cigarete, and why is it plural?

Cigarete is in the accusative plural of cigareta (cigarette).

Nominative singular: cigareta
Accusative singular: cigaretu
Nominative plural: cigarete
Accusative plural: cigarete

It’s the direct object of the verb pušiti (to smoke):
(tko?) ljudi – (što?) cigarete.

It’s plural because the sentence talks about people smoking cigarettes in general, not a single cigarette:

pušiti cigaretu – to smoke a cigarette (one)
pušiti cigarete – to smoke cigarettes (in general, habitually).

How do you know whether grad means the city or a city if Croatian has no articles?

Croatian has no words like the or a/an, so grad by itself can correspond to any of:

the city
a city
the town / a town (depending on context)

Which one you choose in English depends on context and what sounds natural.

In your sentence, English would usually use the because we’re talking about a specific city (the one we are describing):

Grad je manje zagađen… → The city is less polluted…

Why is it na ulici, and not u ulici or na ulicu?

Prepositions and cases:

na + locative → location / “on, at, in (open public place)”
na + accusative → direction / “onto, to”

Here we describe location (where they smoke), so we use na + locative:

na ulici (locative → ulica → ulici) = on the street / in the street

Compare:

Puše cigarete na ulici. – They smoke cigarettes in the street. (static location)
Izlaze na ulicu. – They go out onto the street. (movement to the street)

U ulici is usually not used; u tends to be used with enclosed spaces: u kući (in the house), u sobi (in the room), etc., while na ulici is the normal phrase for in the street.

Is the word order pušiti cigarete na ulici fixed, or could I say pušiti na ulici cigarete?

The neutral, most common order is:

pušiti cigarete na ulici (verb – object – adverbial)

You can say pušiti na ulici cigarete, and it is grammatically correct, but it sounds slightly marked or emphatic, as if you were contrasting “on the street” with some other place.

Basic guideline: for neutral sentences, keep:

[verb] + [object] + [place/time phrase]
→ pušiti cigarete na ulici

Why is it ljudi and not something like čovjeci? What form is ljudi?

The basic noun is:

čovjek – person, man, human being

Its plural is irregular:

nominative singular: čovjek
nominative plural: ljudi (not čovjeci in modern standard language)

In your sentence:

ljudi is nominative plural (the subject): Ljudi prestanu pušiti…

So:

ljudi = people
It’s always plural; there is no singular ljud.

Should there be a comma before kad in this sentence?

In this word order, no comma is needed or usual:

Grad je manje zagađen kad ljudi prestanu pušiti cigarete na ulici. ✅

General pattern:

If the kad-clause comes first, you use a comma:
Kad ljudi prestanu pušiti cigarete na ulici, grad je manje zagađen.
If the kad-clause comes after the main clause, you normally don’t use a comma:
Grad je manje zagađen kad ljudi prestanu pušiti cigarete na ulici.