Kad temperatura padne ispod nule i pojavi se mraz, cesta je opasnija nego obično.

Both kad and kada mean when.

• kad is the shorter, very common everyday form
• kada is a bit fuller and can sound slightly more formal or emphatic

In this sentence, both would work:

• Kad temperatura padne...
• Kada temperatura padne...

The meaning does not really change.

Because Croatian is using perfective verbs here.

• padne = from pasti (perfective)
• pojavi se = from pojaviti se (perfective)

After kad, Croatian often uses perfective present forms to talk about:

• something that happens at a certain point
• a completed event before the main result
• a general rule: whenever this happens, that happens

So the idea is not while the temperature is falling, but rather when it drops below zero.

If you used imperfective forms like pada or pojavljuje se, the meaning would feel more like an ongoing process or repeated background action, which is not the point here.

It is best understood as a general truth / whenever-situation.

Croatian often uses present-tense forms this way:

• Kad temperatura padne... cesta je opasnija...

This means something like:

• whenever the temperature drops below zero, the road becomes more dangerous

So even though the verbs look like present tense, the sentence is not just about right now. It describes what generally happens under those conditions.

Because ispod requires the genitive case.

The noun nula becomes nule in the genitive singular:

• nominative: nula
• genitive: nule

So:

• ispod nule = below zero

This is a standard pattern with ispod:

• ispod stola = under the table
• ispod mosta = under the bridge
• ispod nule = below zero

Because the expression is about zero as a single reference point.

Ispod nule is an idiomatic weather expression meaning below zero. Croatian treats nula here as one value, so singular is exactly what you expect.

Se is a clitic: a short unstressed word that cannot stand freely in just any position.

In Croatian, clitics usually go near the beginning of their clause, but not in a strongly stressed position by themselves. That is why pojavi se is natural here.

So:

• i pojavi se mraz = natural

You may also see other word orders in other contexts, such as:

• kad se pojavi mraz
• mraz se pojavi

But in this sentence, pojavi se mraz is a normal and natural arrangement.

Because one kad can introduce both coordinated actions.

So this structure is normal:

• Kad temperatura padne ispod nule i pojavi se mraz, ...

It means:

• when the temperature drops below zero and frost appears, ...

Croatian does not need to repeat kad before the second verb if both actions belong to the same when-clause.

You could repeat it:

• Kad temperatura padne ispod nule i kad se pojavi mraz...

That is also correct, but it is a bit heavier and less compact.

Mraz is in the nominative singular because it is the subject of pojavi se.

So in the second clause:

• pojavi se mraz
• mraz = the thing that appears

Likewise, in the first clause:

• temperatura padne
• temperatura is the subject there

Because opasnija must agree with cesta, and cesta is feminine singular.

The base adjective is:

• opasan = dangerous

Its comparative is:

• masculine: opasniji
• feminine: opasnija
• neuter: opasnije

Since cesta is feminine, you need:

• cesta je opasnija

If the noun were masculine, you would use opasniji:

• put je opasniji

It is the comparative form of opasan.

• positive: opasan = dangerous
• comparative: opasniji / opasnija / opasnije = more dangerous

Croatian often forms comparatives with endings like -iji. Then the adjective changes form to match the gender and number of the noun.

Here:

• cesta = feminine singular
• therefore: opasnija

Because nego is the normal word used after a comparative to mean than.

So:

• opasnija nego obično = more dangerous than usual

This is the standard comparison pattern:

• veći nego prije = bigger than before
• brži nego inače = faster than usual
• opasnija nego obično = more dangerous than usual

Here obično is functioning as an adverb, not as an adjective.

It means:

• usually
• ordinarily
• in this sentence, effectively than usual

So:

• nego obično = than usual

It is not describing a noun directly. It is part of the comparison.

A similar alternative you might also hear is:

• nego inače = than usual / than otherwise

Not necessarily. Croatian often uses the singular to make a general statement about something as a type.

So cesta here can mean:

• the road, in a general sense
• a road under those conditions

It does not have to mean one particular road only.

A plural version is also possible:

• ceste su opasnije nego obično

That would mean roads are more dangerous than usual. But the singular is very natural for a general statement.

Because the sentence begins with a dependent clause introduced by kad, and then moves to the main clause.

Structure:

• Kad temperatura padne ispod nule i pojavi se mraz, = dependent clause
• cesta je opasnija nego obično. = main clause

Croatian normally separates that opening kad-clause from the main clause with a comma.