Na karoserii je koroze.

Here na expresses a location: something is on the car body / bodywork.

With na, Czech changes case depending on meaning:

• location → usually locative
• movement toward a surface/place → usually accusative

So in Na karoserii je koroze, this is a static location, so Czech uses the locative:

• na karoserii = on the bodywork

Compare:

• Je to na karoserii. = It is on the bodywork.
• Dám nálepku na karoserii. = I’ll put a sticker onto the bodywork.

In the second example, the form happens to look the same, but the grammar function is different.

Because karoserii is a declined form of the noun karoserie.

• dictionary form: karoserie
• after na for location: na karoserii

So the ending changes because Czech nouns change form according to case.

A useful way to think about it:

• karoserie = the basic form
• na karoserii = on the bodywork

For many feminine nouns ending in -ie, the locative singular ends in -ii.

Koroze is in the nominative singular.

It is the thing that is present in the sentence. Czech often uses this structure:

• [place] + je + [thing in nominative]

So literally:

• Na karoserii je koroze.
• On the bodywork is corrosion.

Natural English says There is corrosion on the bodywork, but Czech does not need a separate word like English there in this pattern.

Yes. Je is the 3rd person singular of být (to be), so it means is.

• je = is

In this sentence, Czech uses je to express existence/presence, much like English there is:

• Na karoserii je koroze. = There is corrosion on the bodywork.

A key difference from English: Czech does not need a dummy word like there. The verb je is enough.

Because Czech has no articles.

English distinguishes:

• a body
• the body

Czech usually does not. Context tells you whether English would use a, the, or no article at all.

So:

• Na karoserii je koroze

can correspond to English with the because the context makes it definite: the bodywork.

Yes. Czech word order is fairly flexible, and changing it often changes the focus or emphasis, not the basic meaning.

• Na karoserii je koroze.
  Natural when you are talking about the bodywork and saying what is on it.

• Koroze je na karoserii.
  More emphasis on koroze or a contrast like The corrosion is on the bodywork.

The version with the place first is very natural for introducing what is present somewhere.

Koroze is correct, but it sounds a bit more technical or formal, like corrosion in English.

In everyday speech, many speakers would often say rez for rust:

• Na karoserii je rez.

So:

• koroze = corrosion, somewhat more technical/broader
• rez = rust, everyday word for the rust you see on metal

If you are talking about visible rust on a car, rez is often the more conversational choice.

The main stress in Czech is usually on the first syllable of the word.

So:

• karoserii → stress on ka
• roughly: KA-ro-se-ri-i

The final ii is not like one English sound. It is basically two i sounds next to each other, so you should pronounce both parts clearly.

Also:

• koroze → KO-ro-ze

Czech pronunciation is usually more regular than English spelling, so once you know the sound rules, words are quite predictable.