Kupac nije proverio ni registraciju ni osiguranje.

Because Serbian normally makes the negative of the past tense with the negative form of biti (to be) plus the past participle.

So:

• je proverio = he checked / has checked
• nije proverio = he did not check / has not checked

The word nije is the negative form of je here.
So the structure is:

• kupac = the buyer
• nije = did not / has not
• proverio = checked

This is the standard way to negate this tense in Serbian.

The ending -o shows that the subject is masculine singular.

The subject here is kupac (buyer), which is a masculine noun, so the past participle agrees with it:

• masculine singular: proverio
• feminine singular: proverila
• neuter singular: proverilo
• masculine plural: proverili
• feminine plural: proverile

So if the subject were a woman, you would say:

• Kupac is masculine, so: Kupac nije proverio...
• If it were kupkinja or žena, then: ...nije proverila...

Ni ... ni ... means neither ... nor.

So:

• ni registraciju ni osiguranje = neither the registration nor the insurance

In Serbian, this often appears together with a negative verb, so the sentence literally has a kind of double negative, which is completely normal:

• nije proverio ni registraciju ni osiguranje

A natural English translation is:

• The buyer checked neither the registration nor the insurance
• or more naturally: The buyer didn’t check either the registration or the insurance

Because Serbian normally uses negative concord. That means multiple negative elements appear together in the same sentence.

So Serbian says, literally:

• The buyer did not check neither registration nor insurance

But in correct English, you usually change that to:

• The buyer didn’t check either the registration or the insurance
• or The buyer checked neither the registration nor the insurance

In Serbian, using both nije and ni ... ni ... is not a mistake. It is the normal pattern.

Because registraciju is in the accusative singular, which is the case used here for a direct object.

The noun is:

• nominative: registracija
• accusative: registraciju

Since the buyer did not check what? — the registration — Serbian uses the accusative:

• proveriti registraciju

This is a feminine noun ending in -a, so in the singular accusative it changes to -u.

Because osiguranje is also a direct object, so it is in the accusative singular — but for this noun, the accusative looks the same as the nominative.

This noun is neuter, and many neuter singular nouns have the same form in nominative and accusative:

• nominative: osiguranje
• accusative: osiguranje

So:

• proveriti osiguranje = to check the insurance

That is why registraciju changes form, but osiguranje does not.

Kupac means buyer or customer, depending on context.

In this sentence, buyer is probably the best translation, especially if the context is something like buying a car and checking its paperwork.

Grammatically:

• kupac is masculine singular
• it is in the nominative case because it is the subject of the sentence

So:

• Kupac = The buyer
• Kupac nije proverio... = The buyer did not check...

This is the past tense, more specifically the common Serbian past tense often called the perfect.

It is formed with:

• a present-tense form of biti (to be)
• plus the past participle

Here:

• nije = negative form of the auxiliary
• proverio = past participle

So nije proverio means:

• did not check
• or sometimes has not checked, depending on context

In many everyday situations, English will translate it simply as did not check.

Proveriti is perfective.

That matters because perfective verbs usually present an action as a completed whole. So nije proverio means that the action of checking was not carried out / not completed.

The imperfective partner is usually proveravati or sometimes proveravati/proveravati depending on usage and context.

Compare:

• nije proverio = he did not check / he failed to check
• nije proveravao = he was not checking / he did not use to check / he wasn’t checking repeatedly

In this sentence, the perfective verb is the natural choice because it refers to a specific completed action that did not happen.

Yes, but it would mean something slightly different.

• nije proverio focuses on a single completed check that did not happen
• nije proveravao sounds more ongoing, repeated, habitual, or process-oriented

So:

• Kupac nije proverio ni registraciju ni osiguranje
  = The buyer didn’t check either the registration or the insurance

• Kupac nije proveravao ni registraciju ni osiguranje
  = The buyer wasn’t checking either the registration or the insurance
  or The buyer did not check the registration or insurance as a practice / over a period of time

For one concrete event, nije proverio is the better choice.

Yes. Serbian word order is fairly flexible, although some versions sound more natural than others depending on emphasis.

The neutral version is:

• Kupac nije proverio ni registraciju ni osiguranje.

You might also hear:

• Ni registraciju ni osiguranje kupac nije proverio.

That version puts extra emphasis on the two things that were not checked.

Even when the order changes, the case endings help show what each word is doing in the sentence.

No. Serbian does not have articles like a/an/the.

So:

• kupac can mean a buyer or the buyer
• registraciju can mean a registration or the registration
• osiguranje can mean insurance or the insurance

The exact meaning depends on context.

In English, you usually have to add articles, but Serbian leaves that information unstated unless the context makes it clear.

In many real-life contexts, especially with cars, registracija often means the vehicle registration or registration papers/status, not just the abstract act of registering.

So if this sentence is about a car purchase, registraciju most likely means something like:

• the car’s registration
• registration documents
• whether the vehicle is properly registered

Likewise, osiguranje would likely mean:

• the vehicle’s insurance
• the insurance policy/status

So the sentence may imply that the buyer failed to check important paperwork.

A simple pronunciation guide is:

• Kupac ≈ KOO-pats
• nije ≈ NEE-ye
• proverio ≈ pro-VE-ri-o
• ni ≈ nee
• registraciju ≈ re-gi-stra-TSI-yu
• osiguranje ≈ o-si-GOO-ra-nye

A rough full pronunciation:

• KOO-pats NEE-ye pro-VE-ri-o nee re-gi-stra-TSI-yu nee o-si-GOO-ra-nye

A few helpful sounds:

• c in kupac sounds like ts
• j sounds like English y
• nj in osiguranje sounds like the ny in canyon

Yes. Proveriti can mean check, verify, or inspect, depending on context.

Possible translations include:

• The buyer didn’t check either the registration or the insurance.
• The buyer verified neither the registration nor the insurance.
• The customer did not check the registration or the insurance.

Among these, check is usually the most natural everyday English choice. Verify sounds a bit more formal.