Pločnik je opasniji kad se pojavi mraz.

Because pločnik is the subject of the sentence.

• pločnik = sidewalk / pavement
• It is the thing being described as opasniji (more dangerous)

In Croatian, the subject of a sentence is normally in the nominative case, so pločnik stays in its basic dictionary form.

je is the 3rd person singular present-tense form of the verb biti (to be).

So:

• Pločnik je opasniji = The sidewalk is more dangerous

It links the subject pločnik with the description opasniji.

opasniji is the comparative form of opasan (dangerous).

• opasan = dangerous
• opasniji = more dangerous

It agrees with pločnik, which is:

• masculine
• singular
• nominative

So the masculine singular nominative form is opasniji.

You can compare:

• opasan pločnik = a dangerous sidewalk
• opasniji pločnik = a more dangerous sidewalk

In this sentence, it is used predicatively after je:

• Pločnik je opasniji = The sidewalk is more dangerous

Because the sentence expresses a comparison, even if the thing being compared is only understood from context.

opasan would mean simply dangerous.
opasniji means more dangerous.

So the idea is something like:

• The sidewalk is more dangerous when frost appears
• in other words, more dangerous than usual / more dangerous than before

If you said Pločnik je opasan kad se pojavi mraz, that would mean The sidewalk is dangerous when frost appears, which is slightly different.

Both mean when.

• kad = shorter, very common in everyday speech
• kada = fuller form, sometimes a bit more formal or emphatic

So:

• kad se pojavi mraz
• kada se pojavi mraz

Both are correct here.

Here se is a reflexive particle that is part of the verb expression pojaviti se, which means:

• to appear
• to show up
• to emerge

So:

• pojaviti on its own is less likely in this meaning
• pojaviti se is the normal form for appear

That means:

• kad se pojavi mraz = when frost appears / when frost forms / when frost shows up

English learners often want to translate se literally as self, but in many Croatian verbs it is simply part of the normal verb form and is not translated word-for-word.

This is about aspect.

• pojaviti se = perfective
• pojavljivati se = imperfective
• pojavljuje se = present tense of the imperfective verb

In clauses like kad se pojavi mraz, Croatian often uses the perfective present to mean when something happens / when it appears.

So:

• kad se pojavi mraz = when frost appears / once frost appears

This sounds natural for an event that happens as a whole.

If you used kad se pojavljuje mraz, it would sound unusual here, because that suggests an ongoing or repeated process in a way that does not fit this sentence as well.

Because mraz is the subject of the clause kad se pojavi mraz.

That clause literally works like:

• mraz se pojavi = frost appears

Since mraz is the thing doing the appearing, it is in the nominative case.

Literally, it is closer to when frost appears or when frost sets in.

But in natural English, depending on context, it may be translated more smoothly as:

• when frost appears
• when frost forms
• when there is frost

The Croatian wording focuses on the arrival/emergence of frost, not just its static existence.

Yes. Croatian word order is fairly flexible.

For example, you could also say:

• Kad se pojavi mraz, pločnik je opasniji.

This means the same thing.

The original version:

• Pločnik je opasniji kad se pojavi mraz.

puts the main statement first, then adds the time/condition clause.

The reordered version puts more focus on when frost appears.

Yes. In Croatian, kad can mean both:

• when in a specific situation
• whenever in a general truth or repeated situation

In this sentence, it most naturally sounds like a general statement:

• Whenever frost appears, the sidewalk is more dangerous.

So even though English may use when, the Croatian can easily carry the idea of whenever.

A rough pronunciation is:

• PLOCH-neek

A few useful details:

• č sounds like ch in chocolate
• nj sounds like the ny sound in canyon for many learners, though the exact Croatian sound is a single consonant
• stress is typically on the first syllable here: ploč-

So pločnik is approximately PLOCH-nik, with a short i in the last syllable.

Here it is treated like a normal masculine noun meaning frost.

In this sentence, it behaves grammatically like a regular singular noun:

• mraz = nominative singular

Weather words in Croatian are often used as ordinary nouns, so this is completely normal:

• pojavio se mraz = frost appeared
• nema mraza = there is no frost

So yes, even though English often treats weather terms in special ways, Croatian grammar handles mraz as a straightforward noun.

Yes, you could, and it would slightly change the nuance.

• Pločnik je opasniji kad se pojavi mraz = The sidewalk is more dangerous when frost appears.
• Pločnik postaje opasniji kad se pojavi mraz = The sidewalk becomes more dangerous when frost appears.

The version with je states the condition more simply.
The version with postaje emphasizes the change into a more dangerous state.

Both are natural, but the original sentence is simpler and more neutral.