Pljeskavice i ćevapi su na rešetki.

The singular forms are:

• pljeskavica → pljeskavice in the plural
• ćevap → ćevapi in the plural

So the sentence is talking about more than one grilled patty and more than one ćevap.

Su is the 3rd person plural form of the verb biti meaning to be.

Because the subject is plural — pljeskavice i ćevapi — Serbian uses the plural verb:

• je = is
• su = are

So Pljeskavice i ćevapi su... means The pljeskavices and ćevapi are...

They are in the nominative plural, because they are the subject of the sentence.

• pljeskavice = nominative plural
• ćevapi = nominative plural

In Serbian, the subject of a sentence is normally in the nominative case.

Because after na when it means on in the sense of location, Serbian uses the locative case.

The basic form is:

• rešetka = grill grate / rack

But after na for location, it becomes:

• na rešetki = on the grill grate / on the rack

So:

• na rešetki = location
• not na rešetka, because that would be the dictionary form, not the correct case after the preposition

It is the locative singular of rešetka.

This happens because:

• na
  • location usually takes the locative
• rešetka is singular
• therefore: na rešetki

So the structure is:

• na
  • locative singular

Their genders are:

• pljeskavica — feminine
• ćevap — masculine
• rešetka — feminine

This helps explain some of the endings:

• feminine nouns like pljeskavica often form the plural in -e
• masculine nouns like ćevap often form the plural in -i

The word order is somewhat flexible.

The neutral order here is:

• Pljeskavice i ćevapi su na rešetki.

But Serbian can also move things around for emphasis, for example:

• Na rešetki su pljeskavice i ćevapi.

That would put more focus on where they are.

So the original order is natural and straightforward, but not the only possible one.

In standard Serbian, you normally do need su here.

Unlike some languages where the present tense of to be can be dropped, Serbian usually keeps it:

• Pljeskavice i ćevapi su na rešetki. = correct
• Pljeskavice i ćevapi na rešetki. = possible only in very informal, headline-like, or fragmented speech

So for normal, correct sentence building, keep su.

A rough guide:

• ć in ćevapi: a soft sound somewhat like t
  • y, softer than English ch
• š in rešetki: like English sh
• lj in pljeskavice: like a soft ly sound

Very roughly:

• pljeskavice ≈ plyes-ka-vee-tse
• ćevapi ≈ tye-va-pee
• rešetki ≈ re-SHET-kee

These are only approximations, but they are useful for getting started.

Because Serbian does not have articles like English the or a/an.

So:

• pljeskavice can mean pljeskavices, the pljeskavices, or sometimes some pljeskavices
• ćevapi can mean ćevapi, the ćevapi, or some ćevapi

The exact meaning depends on context, not on an article.

Not always, but very often it does.

In this sentence, na means on, because it describes location:

• na rešetki = on the grill grate

A useful pattern is:

• na
  • locative = on / at, showing location
• na
  • accusative = onto / to, often showing movement toward a surface or place

Here there is no movement, only location, so na rešetki is locative.

Not necessarily by grammar alone.

The sentence literally states their location:

• Pljeskavice i ćevapi su na rešetki. = The pljeskavices and ćevapi are on the grill grate.

In real-life context, that often suggests they are currently being grilled, but the grammar itself only says where they are. The idea of an ongoing action comes from context, not from a special verb form.