Stomatolog buši zub sa karijesom.

Buši is the 3rd person singular present tense of the verb bušiti.

So:

• ja bušim = I drill
• ti bušiš = you drill
• on/ona buši = he/she drills

Here it matches stomatolog, which is singular, so we get Stomatolog buši...

Also, bušiti is imperfective, so it presents the action as ongoing, repeated, or simply as an action in progress.

Because zub is the direct object of the verb, so it is in the accusative case.

However, zub is a masculine inanimate noun, and in Serbian, masculine inanimate nouns usually have the same form in the accusative as in the nominative.

So:

• nominative: zub
• accusative: zub

Compare that with a masculine animate noun:

• pas = dog
• accusative: psa

That is why you see zub, not zuba.

Karijesom is in the instrumental singular.

The preposition sa / s usually takes the instrumental when it means with.

So:

• karijes = tooth decay / cavity
• sa karijesom = with tooth decay / with a cavity

That is why the noun changes to karijesom.

A useful extra point: the same preposition can take a different case in other meanings. For example, when sa means from/off, it takes the genitive:

• sa stola = from the table / off the table

It describes the tooth.

So the structure is:

• Stomatolog = subject
• buši = verb
• zub sa karijesom = object phrase

In other words, it means the dentist is drilling the tooth that has decay.

If you want to make that relationship more explicit, Serbian can also say:

• Stomatolog buši zub koji ima karijes.

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

So zub can mean:

• a tooth
• the tooth

The exact meaning depends on context.

If Serbian speakers want to make it more specific, they can use other words, for example:

• taj zub = that tooth
• jedan zub = one / a certain tooth

Yes. Serbian word order is more flexible than English because case endings show what each word is doing.

The neutral, most straightforward order here is:

• Stomatolog buši zub sa karijesom.

But other orders are possible for emphasis, for example:

• Zub sa karijesom buši stomatolog.

That sounds more marked and would usually be used only in a specific context, for contrast or emphasis. For a learner, the original word order is the best default.

Because Serbian often expresses this kind of idea with a noun + prepositional phrase, where English might prefer an adjective.

So zub sa karijesom is a normal way to say a tooth with decay / a tooth with a cavity.

Serbian can also use other expressions depending on style, for example:

• pokvaren zub = a bad / decayed tooth
• zub koji ima karijes = a tooth that has decay

So the Serbian structure is natural; it just works differently from English.

Yes. Sa and s are two forms of the same preposition meaning with.

In Serbian, sa is often used when it sounds smoother or is preferred in everyday speech. In this sentence, sa karijesom simply means with tooth decay.

So learners should recognize both as the same basic preposition:

• s
• sa