Pametan prodavac vidi prevaru.

Because adjectives in Serbian must agree with the noun they describe in gender, number, and case.

Here, prodavac is masculine singular nominative, so the adjective is pametan.

Compare:

• pametan prodavac = masculine
• pametna prodavačica = feminine
• pametno dete = neuter

Because it is the direct object of vidi, so it takes the accusative case.

The basic dictionary form is prevara. But many feminine nouns ending in -a change to -u in the accusative singular:

• žena → ženu
• knjiga → knjigu
• prevara → prevaru

You know mainly from the case endings.

• prodavac is in the nominative, which is the usual case for the subject
• prevaru is in the accusative, which is the usual case for a direct object

So even if you did not rely on word order, the endings tell you who is doing the action and what is being seen.

Vidi is the 3rd person singular present tense form of videti.

Present tense:

• ja vidim
• ti vidiš
• on/ona/ono vidi
• mi vidimo
• vi vidite
• oni/one/ona vide

It matches the singular subject prodavac.

Because Serbian normally has no articles.

So Pametan prodavac vidi prevaru can mean something like:

• A smart salesman sees a fraud
• The smart salesman sees the fraud
• or another article choice, depending on context

English forces you to choose a/an or the. Serbian usually leaves that to context.

Because pametan is directly modifying the noun prodavac. It is part of the noun phrase pametan prodavac.

You would use je if the adjective were the predicate:

• Prodavac je pametan. = The salesman is smart.

But in your sentence, the structure is simply:

• smart salesman sees fraud

So no je is needed.

Yes. Serbian word order is more flexible than English because case endings carry a lot of the grammar.

The sentence as given is the most neutral:

• Pametan prodavac vidi prevaru.

You could also say:

• Prevaru vidi pametan prodavac.

That still means basically the same thing, but it gives more emphasis to prevaru.

Grammatically, prodavac is a masculine noun, so it normally refers to a male seller or salesman.

If you want to refer specifically to a female seller, the usual word is prodavačica.

No. Prevara can mean fraud, deception, a trick, a scam, and in some contexts even cheating or infidelity.

So the exact nuance depends on context. In this sentence, it most naturally means some kind of deception or scam that the seller notices.

No. Serbian uses both Latin and Cyrillic.

In Cyrillic, this sentence is: Паметан продавац види превару.

The grammar and meaning stay the same.

A rough English-friendly pronunciation is:

• pametan ≈ pah-meh-tahn
• prodavac ≈ pro-dah-vahts
• vidi ≈ vee-dee
• prevaru ≈ preh-vah-roo

A few useful notes:

• c is pronounced like ts
• Serbian spelling is quite phonetic, so words are usually pronounced much as they are written
• stress is not normally marked in everyday writing