Ukras i konfete su na stolu.

Because the subject is compound: ukras i konfete = the decoration and the confetti.

In Serbian, when two things are joined by i (and), the verb normally goes in the plural:

• ukras je = the decoration is
• konfete su = the confetti are
• ukras i konfete su = the decoration and the confetti are

So su is the 3rd person plural form of biti (to be).

Because after na meaning on in a location sense, Serbian uses the locative case.

The basic form is:

• sto = table

But after na when something is already located on the table, it becomes:

• na stolu = on the table

So:

• sto = nominative
• stolu = locative

This is one of the most important patterns to learn:

• na stolu = on the table
• u kući = in the house
• na polici = on the shelf

Because Serbian distinguishes between location and movement toward a destination.

• na stolu = on the table, already there → locative
• na sto = onto the table, movement toward it → accusative

So compare:

• Ukras je na stolu. = The decoration is on the table.
• Stavljam ukras na sto. = I am putting the decoration onto the table.

In your sentence, the decoration and confetti are already located there, so na stolu is correct.

Here is the breakdown:

• ukras — nominative singular
• konfete — nominative plural
• su — verb, 3rd person plural
• na stolu — prepositional phrase with locative

The subject of the sentence is ukras i konfete, so those words are in the nominative.
The phrase na stolu shows location, so stolu is in the locative.

In this sentence, konfete is plural.

The singular form is:

• konfeta = one piece of confetti

The plural is:

• konfete = confetti

So that is another reason the verb must be plural in the full sentence.

Because Serbian does not have articles like a and the.

English says:

• the decoration
• the table

Serbian simply says:

• ukras
• sto / stolu

Whether something means a decoration, the decoration, decorations, or the decorations is usually understood from context.

So Ukras i konfete su na stolu can naturally mean:

• The decoration and the confetti are on the table or, depending on context,
• A decoration and confetti are on the table

Yes. Serbian word order is much more flexible than English word order.

Your sentence:

• Ukras i konfete su na stolu.

A very natural alternative is:

• Na stolu su ukras i konfete.

Both are correct. The difference is mostly about focus:

• Ukras i konfete su na stolu. → focus starts with the things
• Na stolu su ukras i konfete. → focus starts with the location

Serbian often moves elements around for emphasis or style, while still keeping the same basic meaning.

Ukras is masculine.

You can often tell because many masculine nouns in Serbian end in a consonant in the nominative singular:

• ukras = decoration
• grad = city
• prozor = window

That said, in this sentence the gender of ukras does not change the form of the verb, because the whole subject is plural: ukras i konfete su.

Because ukras is in the nominative singular, which is its basic dictionary form.

In this sentence:

• ukras is one part of the subject
• konfete is the other part of the subject

Only stolu changes because it is inside the phrase na stolu, where the preposition na requires the locative for location.

So:

• ukras stays ukras
• sto changes to stolu

In standard Serbian, in a normal present-tense sentence like this, su is usually kept:

• Ukras i konfete su na stolu.

Leaving it out would sound incomplete or nonstandard in ordinary usage.

So unlike some languages where the present tense of to be can be omitted, Serbian normally uses it.

A rough pronunciation guide is:

• Ukras → OO-krahs
• i → ee
• konfete → kon-FEH-teh
• su → soo
• na → nah
• stolu → STOH-loo

Put together:

OO-krahs ee kon-FEH-teh soo nah STOH-loo

The letter-to-sound relationship in Serbian is quite regular, much more regular than in English.

It follows a very common Serbian structure:

subject + biti + location

So here:

• Ukras i konfete = subject
• su = form of biti (to be)
• na stolu = location

This pattern is extremely useful:

• Knjiga je na stolu. = The book is on the table.
• Ključevi su u torbi. = The keys are in the bag.
• Deca su u parku. = The children are in the park.

So your sentence is a very typical location sentence in Serbian.