Na stolu je torta, a na torti su sveće.

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

So:

• sto → na stolu
• torta → na torti

The basic forms you see in dictionaries are usually nominative singular, but prepositions often require other cases.

They belong to different noun patterns.

• sto is a masculine noun, and masculine/neuter nouns often take -u in the locative singular: na stolu
• torta is a feminine noun ending in -a, and these often take -i in the locative singular: na torti

So the case is the same, but the ending depends on the noun type.

Not always. With na, the case depends on whether you mean location or movement toward something.

• location → locative
  Na stolu je torta. = the cake is on the table
• movement/destination → accusative
  Stavljam tortu na sto. = I am putting the cake onto the table

This is a very important Serbian pattern:

• where? → locative
• where to? → accusative

Serbian often expresses there is / there are with the verb biti (to be) plus word order and context.

So:

• Na stolu je torta literally looks like On the table is a cake
• Na torti su sveće literally looks like On the cake are candles

This is a normal Serbian way to present the existence of something in a place. Serbian does not need a separate dummy word like English there.

Both are correct, but they emphasize things differently.

• Na stolu je torta focuses first on the location, then introduces what is there
• Torta je na stolu starts with the cake and says where it is

In this sentence, the speaker is probably painting a scene step by step:

• on the table, there is a cake
• on the cake, there are candles

So the location-first order sounds very natural.

Also, je and su are short unstressed forms called clitics, and they often appear near the beginning of the clause.

Here a is best understood as and, but with a slight sense of contrast, switch of focus, or meanwhile.

So it connects the two clauses smoothly:

• first clause: on the table is a cake
• second clause: and on the cake are candles

It is not a strong contrast like ali (but), but it does signal a shift from one detail to another.

Because Serbian has no articles.

English distinguishes:

• a cake
• the cake

Serbian usually leaves that to context:

• torta can mean a cake or the cake
• sveće can mean candles or the candles

If needed, Serbian can add other words for clarity, such as:

• jedna torta = a/one cake
• ta torta = that/the cake

But normally, no article is used.

Because the verb agrees with the subject in number.

• torta is singular, so Serbian uses je
• sveće is plural, so Serbian uses su

So:

• torta je / je torta
• sveće su / su sveće

The position can change, but the agreement must stay correct.

The basic singular form is sveća, meaning candle.

Its plural nominative form is sveće.

So:

• singular: sveća
• plural: sveće

That is why the sentence uses su: the noun is plural.

Yes, in normal Serbian punctuation, a comma before a is standard when it joins two clauses like this.

So:

• Na stolu je torta, a na torti su sveće.

This is the expected written form.

Approximately SVEH-cheh, but with an important detail: ć is softer than English ch.

A rough guide:

• sv as in sve-
• e like e in bet
• ć is a soft sound, somewhere between English t and ch
• final e is clearly pronounced

So sveće is not exactly English sveche, but that approximation can help at first.

The letter ć is softer than č, which English speakers often find tricky.