Learn Hardcore Croatian: Preko mosta hodamo oprezno jer je promet opasan. - Over the bridge we walk carefully because the traffic is dangerous.

Because preko takes the genitive case. Mosta is the genitive singular of most (bridge). Minimal forms to see the pattern: N most, G mosta, D/L mostu, A most, I mostom, V moste. With preko + genitive, you express “across/over (to the other side of)” something.

Could I say na mostu, po mostu, or kroz most instead of preko mosta?

preko mosta: across the bridge, from one side to the other (that’s what you want here).
na mostu: on the bridge (location), e.g., We are on the bridge.
po mostu: along/over the surface of the bridge, moving around on it.
kroz most: through the bridge (would imply an interior, like a tunnel)—normally wrong for a standard bridge.

Why is there no word for “the” before “bridge” or “traffic”?

Croatian has no articles. Specificity is shown by context or demonstratives. If you need “that bridge,” say taj/ovaj/onaj most. If you need to specify the traffic, you can say taj promet or add something like današnji promet (today’s traffic).

What does hodamo tell me about the subject? Why is “we” not written?

Verb endings encode person and number, so Croatian usually drops subject pronouns. Hodamo = we walk. You can add mi (we) for emphasis or contrast: Mi hodamo oprezno…

Could I use idemo, šetamo, or pješačimo instead of hodamo?

hodamo: we are walking (neutral).
idemo: we are going (not necessarily on foot).
šetamo: we are strolling (leisurely, more relaxed feel).
pješačimo: we are going on foot/walking (emphasizes on-foot travel, sometimes longer distance).
All can fit, but they change the nuance.

Why is it oprezno and not oprezan?

Oprezno is the adverb form (carefully). In Croatian, many adverbs are identical to the neuter singular of the adjective: oprezan (m), oprezna (f), oprezno (n/adverb).
Compare: Hodamo oprezno (We walk carefully) vs. Promet je opasan (Traffic is dangerous; adjective agreeing with the noun).

Could I say pažljivo instead of oprezno?

Yes. Pažljivo also means carefully. Oprezno leans toward caution due to danger; pažljivo leans toward carefulness/attentiveness or precision. Both work here.

Why is it jer je promet opasan and not jer promet je opasan?

Clitic verbs like je tend to appear in second position in their clause. After jer (because), the standard placement is jer je…. Jer promet je opasan sounds off in Croatian; say jer je promet opasan.

Can I use zato što instead of jer?

Yes: … jer je promet opasan ≈ … zato što je promet opasan. Jer is short and neutral. Zato što is a bit more explicit/emphatic (“for the reason that”) and slightly heavier in style; both are fine.

Why is it opasan and not opasno after promet?

Agreement. Promet is masculine singular, so the predicate adjective is opasan (masc. sg.). Opasno would either be an adverb or a neuter predicate used impersonally (e.g., Ovdje je opasno = It’s dangerous here).

How flexible is the word order here?

Quite flexible, with changes in emphasis:

Hodamo oprezno preko mosta jer je promet opasan.
Oprezno hodamo preko mosta jer je promet opasan.
Preko mosta hodamo oprezno jer je promet opasan.
Jer je promet opasan, preko mosta hodamo oprezno.
All are acceptable; fronting a part highlights it.

Is promet always the word for “traffic”? What about saobraćaj?

In Croatia, promet is standard. In Serbia and Bosnia, saobraćaj is used. If you’re learning Croatian (HR standard), prefer promet.

Could I say mostom instead of preko mosta?

You can: Hodamo mostom can mean “We walk across/along the bridge,” using the bridge as the route. Preko mosta explicitly highlights crossing to the other side and is the most straightforward choice here. Both are natural, with a slight nuance difference.

How would I make “more carefully” or “very carefully”?

Comparative: opreznije (We walk more carefully = Hodamo opreznije).
Intensifier: vrlo/veoma oprezno, jako oprezno (very carefully).