Learn Hardcore Croatian: Na mostu moramo paziti na promet. - On the bridge we have to pay attention to the traffic.

na – preposition, here meaning on, governs the locative case when talking about location.
mostu – noun most (bridge), masculine singular locative → on the bridge.
moramo – verb morati (to have to, must), 1st person plural, present tense → we must / we have to.
paziti – verb in the infinitive, meaning to pay attention / to be careful / to watch.
na – preposition again, but here it is part of the verb phrase paziti na nešto = to pay attention to something and governs the accusative case.
promet – noun promet (traffic), masculine singular accusative (form is identical to nominative for inanimate masculine nouns).

Literal structure:
On the bridge we-must pay-attention to traffic.

Why is it na mostu and not u mostu?

Croatian distinguishes between:

na
- locative → on a surface / at an open place
  - na mostu = on the bridge (on its surface)
  - na stolu = on the table
u
- locative → in / inside something or in a closed space
  - u autu = in the car
  - u kući = in the house

A bridge is thought of as a surface you are on, not a space you are inside, so standard Croatian says na mostu, not u mostu.
Using u mostu would sound like you are somehow inside the structure of the bridge, which is not what is meant here.

Why do we say mostu and not most after na?

Because na in the sense of location (“on, at”) requires the locative case.

The noun most declines like this (singular):

Nominative: most (the bridge – subject form)
Genitive: mosta
Dative: mostu
Accusative: most
Locative: (o) mostu
Instrumental: mostom

After na (location), you must use locative → na mostu.
Using nominative na most would be ungrammatical in this static, “where?” meaning. (You do use na most – accusative – with motion: Idemo na most. = We’re going onto the bridge.)

Why is it paziti na promet and not just paziti promet?

The verb paziti has two main patterns:

paziti na + accusative → to pay attention to / watch out for
- paziti na promet = to pay attention to the traffic
- paziti na cestu = watch the road
- paziti na djecu = be careful about / keep an eye on the children
paziti + direct object (without na) → to look after / take care of (more concrete)
- paziti dijete = to look after a child (babysit)
- paziti psa = to look after a dog

With an abstract thing like promet (traffic), the natural expression for “pay attention to” is paziti na promet.
Paziti promet would sound wrong or at least very odd in standard Croatian.

Why does na appear twice, and why are the cases different (na mostu vs na promet)?

You’re seeing two different uses of the same preposition na:

na + locative → place / location (answers where?)
- na mostu = on the bridge
  Here na governs the locative, so you get mostu.
paziti na + accusative → a fixed verb-preposition pattern meaning to pay attention to
- paziti na promet = pay attention to the traffic
  Here na is part of the verb phrase paziti na, and it governs the accusative (promet).

So it’s the same preposition, but:

once used in its basic spatial meaning (on the bridge, locative)
once as part of a verb pattern (pay attention to, accusative)

What form is moramo, and how is morati different from trebati?

Moramo is:

verb: morati (to have to, must)
person: 1st person plural (“we”)
tense: present
meaning: we must / we have to

Morati expresses a stronger obligation, similar to English must / have to:

Moramo paziti na promet. = We must pay attention to the traffic.

Trebati is closer to need to / should / ought to, depending on context:

Trebamo paziti na promet. = We need to / should pay attention to the traffic.

Both are common, but:

moramo → stronger, more obligatory
trebamo → milder, more like advice or necessity

Why is there no word like English “to” before paziti (as in “we must *to pay attention”*)?

Croatian does not use a separate word like English to to mark the infinitive.

Infinitive form here is simply paziti.
After modal verbs like morati, htjeti, moći, trebati, you put the infinitive directly:

Examples:

Moramo paziti. = We have to pay attention.
Želim paziti. = I want to pay attention.
Mogu paziti. = I can pay attention.

So where English has must + infinitive with “to” (sometimes without “to”), Croatian just has modal verb + infinitive without anything in between: moramo paziti, not moramo to paziti.

Is the word order fixed, or can I move parts of the sentence around?

The word order is fairly flexible. All of these are grammatically correct:

Na mostu moramo paziti na promet.
– Neutral, slight emphasis on the location (“On the bridge, we must…”).
Moramo paziti na promet na mostu.
– More neutral English-like order; the location comes at the end.
Moramo na mostu paziti na promet.
– Puts moramo (we must) first; na mostu is in the middle but still clear.

What changes is mostly emphasis and rhythm, not basic meaning.
For a learner, the original sentence Na mostu moramo paziti na promet. is perfectly natural and good to imitate.

What is promet exactly (gender, countability), and can I use anything else instead?

Promet:

meaning: traffic (vehicles on the road, movement of cars, etc.)
gender: masculine
number: usually singular, uncountable (like English “traffic”)
case here: accusative singular (same form as nominative for inanimate masculine nouns)

In other standards of the language:

In Serbian and some Bosnian / Montenegrin varieties, the more common word is saobraćaj instead of promet.
In standard Croatian, promet is the usual word.

So in Croatian you normally say:

paziti na promet = pay attention to the traffic

How do I make this sentence negative, and what’s the difference between ne moramo and ne smijemo?

Negation is made by putting ne in front of the verb:

Ne moramo paziti na promet.
= We don’t have to pay attention to the traffic (it’s not necessary).

To say must not / are not allowed to, Croatian usually uses ne smijemo:

Ne smijemo trčati po mostu.
= We mustn’t / are not allowed to run on the bridge.

So:

ne moramo → it’s not required (lack of obligation)
ne smijemo → it’s forbidden / not allowed (prohibition)

For your exact sentence, a natural prohibition would be something like:

Ne smijemo ignorirati promet na mostu.
= We mustn’t ignore the traffic on the bridge.

Can I add mi (“we”) in the sentence, and would that change the meaning?

Yes, you can say:

Mi na mostu moramo paziti na promet.

In Croatian, the subject pronoun (ja, ti, on, mi, vi, oni) is usually dropped, because the verb ending (-mo in moramo) already shows the person and number (“we”).

If you include mi, you give it extra emphasis, something like:

Mi (as opposed to others) must pay attention to the traffic on the bridge.

So:

Na mostu moramo paziti na promet. – neutral
Mi na mostu moramo paziti na promet. – emphasizes we in contrast to someone else.