Konfete su na stolnjaku.

Because konfete is treated as plural in Serbian, so the verb must also be plural.

• su = are
• je = is

So Serbian says Konfete su..., even though English usually says confetti is....

In normal Serbian usage, konfete is usually treated as a plural noun. That is why it takes su.

This is one of those places where Serbian and English organize the idea differently:

• English: confetti is
• Serbian: konfete su

Because after na, Serbian uses different cases depending on the meaning.

Here the meaning is location: the confetti is already lying there.
So na takes the locative:

• stolnjak = dictionary form
• na stolnjaku = on the tablecloth

Compare:

• Konfete su na stolnjaku. = The confetti is on the tablecloth.
• Stavljam konfete na stolnjak. = I am putting confetti onto the tablecloth.

In the second sentence, there is movement onto the surface, so na takes a different case.

The dictionary form is stolnjak.

It is a masculine noun, and in this sentence it appears in the locative singular:

• stolnjak → na stolnjaku

Because na is used for something being on a surface, while u means in/inside.

So:

• na stolnjaku = on the tablecloth
• u stolnjaku would suggest something like being inside the tablecloth, which does not fit here

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

So Konfete su na stolnjaku can mean:

• Confetti is on the tablecloth
• The confetti is on the tablecloth

The exact sense depends on the context.

Yes. Serbian word order is more flexible than English.

You can also say:

• Na stolnjaku su konfete.

That version puts more emphasis on where they are.

One important detail: su is a clitic (an unstressed little word), so it usually goes in second position in the clause.

Usually no, not in a normal full sentence.

Standard Serbian needs the present-tense form of to be here:

• Konfete su na stolnjaku.

Without su, Konfete na stolnjaku sounds more like a label, note, or fragment than a complete standard sentence.

Nj is pronounced as one sound, similar to:

• ny in canyon
• Spanish ñ

So stolnjaku is roughly pronounced stol-nya-koo.

No. A word ending in -e is not automatically neuter singular.

In this sentence, konfete behaves as a plural noun, which you can tell from the verb su. So you should learn konfete as a word that normally goes with plural agreement.