Stomatolog radi u stomatološkoj ordinaciji.

• stomatolog = dentist / stomatologist
• radi = works / is working
• u = in
• stomatološkoj ordinaciji = dental office / dentist’s office

So the sentence literally reads: Dentist works in dental office, which in natural English is The dentist works in a dental office or A dentist works in a dental office.

Serbian does not have articles like English a/an and the.
So stomatolog can mean a dentist or the dentist, depending on context.

The same is true for stomatološka ordinacija: it can mean a dental office or the dental office.

U usually means in or into, depending on the case that follows.

Here it means in, because the sentence describes a location where someone works:

• radi u ordinaciji = works in an office

This is a very common way to talk about where someone works, lives, studies, and so on.

Because after u meaning in for a fixed location, Serbian normally uses the locative case.

The basic form is:

• stomatološka ordinacija = dental office

But in the locative singular, it becomes:

• u stomatološkoj ordinaciji = in a/the dental office

So both words change because:

• ordinacija changes to ordinaciji
• the adjective stomatološka changes to stomatološkoj to match it

In Serbian, adjectives agree with the nouns they describe in:

• gender
• number
• case

Here:

• ordinacija is feminine
• it is singular
• it is in the locative case

So the adjective must match:

• stomatološka ordinacija → basic form
• u stomatološkoj ordinaciji → locative singular feminine

This kind of agreement is a central feature of Serbian grammar.

Radi is the 3rd person singular present tense form of raditi = to work.

The subject is stomatolog, which is singular:

• stomatolog radi = the dentist works

Compare:

• ja radim = I work
• ti radiš = you work
• on/ona radi = he/she works
• stomatolozi rade = dentists work

So radi matches the singular subject stomatolog.

It can mean either, depending on context.

Serbian present tense often covers both:

• works
• is working

In this sentence, the most natural interpretation is a general fact:

• A dentist works in a dental office
• The dentist works in a dental office

If the context were about what someone is doing right now, it could also mean is working.

Yes. Raditi can have several meanings, such as:

• to work
• to do
• to function / operate

But in this sentence, because the subject is a profession and we have a workplace after it, the meaning is clearly to work:

• Stomatolog radi u... = The dentist works in...

No, Serbian word order is more flexible than English word order.

The sentence Stomatolog radi u stomatološkoj ordinaciji is a neutral, natural order.

But other orders are possible, for example:

• U stomatološkoj ordinaciji radi stomatolog.
• Radi stomatolog u stomatološkoj ordinaciji.

These alternatives may sound more marked, emphatic, or context-dependent.
For a learner, the original order is the safest and most neutral.

Both can mean dentist.

• stomatolog is more formal/professional
• zubar is more everyday and conversational

So:

• Stomatolog radi u stomatološkoj ordinaciji sounds a bit more formal
• Zubar radi u zubarskoj ordinaciji would sound more everyday

A learner should recognize both.

Because Serbian distinguishes location and motion partly through case.

• u + locative = in / inside a place
• u + accusative = into a place

Here we have a static location:

• u stomatološkoj ordinaciji = in the dental office

If you wanted motion toward that place, you would use the accusative:

• Ide u stomatološku ordinaciju. = He/She is going into / to the dental office.

So the form of the noun tells you which meaning u has.

Its dictionary form is ordinacija, and nouns ending in -a are very often feminine in Serbian.

That matters because feminine nouns affect:

• adjective endings
• case endings

So:

• stomatološka ordinacija = feminine singular
• u stomatološkoj ordinaciji = feminine singular locative

Not every noun ending pattern is perfectly simple, but -a = usually feminine is a very useful rule.

A few useful letter clues:

• š sounds like sh in ship
• j sounds like y in yes
• c sounds like ts in cats

So approximately:

• stomatološkoj → sto-ma-to-lo-shkoy
• ordinaciji → or-di-na-tsi-yi

That is only an approximation for an English speaker, but it helps. The important thing is to remember that Serbian spelling is quite phonetic: words are usually pronounced much the way they are written.