Svjedok je blizu nadzorne kamere i zna serijski broj.

Because blizu normally takes the genitive case in Croatian.

The basic form is:

• nadzorna kamera = surveillance camera

After blizu it changes to genitive singular:

• blizu nadzorne kamere = near the surveillance camera

Both words change because the adjective must agree with the noun:

• nadzorna kamera → nominative singular
• nadzorne kamere → genitive singular

So this is a case change caused by blizu.

Yes, you can think of blizu here as working like a preposition meaning near/close to.

In Croatian, blizu is commonly used with a noun in the genitive:

• blizu kuće = near the house
• blizu škole = near the school
• blizu nadzorne kamere = near the surveillance camera

So for a learner, the safest rule is:

• blizu + genitive

Je is the 3rd person singular present form of biti = to be.

So:

• Svjedok je blizu... = The witness is near...

English also needs is here, and Croatian does too.

Without je, the sentence would be ungrammatical in standard Croatian.

Here serijski broj is the direct object of zna.

The verb znati means to know, and in this sentence the thing known is serijski broj.

So logically you expect the accusative case. The important point is:

• broj is a masculine inanimate noun
• for masculine inanimate nouns, accusative singular is the same as nominative singular

So:

• nominative: serijski broj
• accusative: serijski broj

That is why it looks unchanged.

Because Croatian distinguishes between different kinds of know.

Use znati for:

• facts
• information
• how to do something

So:

• zna serijski broj = knows the serial number
• zna odgovor = knows the answer
• zna plivati = knows how to swim

A different verb, poznavati / poznati, is used for being familiar with people or places:

• Poznajem tog čovjeka. = I know that man.
• Poznaje grad. = He knows the city.

Since a serial number is a piece of information, znati is correct.

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

So:

• svjedok can mean a witness or the witness
• serijski broj can mean a serial number or the serial number

The exact meaning depends on context.

That is completely normal in Croatian.

Because adjectives in Croatian must agree with the noun they describe in:

• gender
• number
• case

Here:

1. nadzorne kamere

   • noun: kamera
   • feminine singular
   • genitive because of blizu
   • so the adjective becomes nadzorne

2. serijski broj

   • noun: broj
   • masculine singular
   • accusative object, but same form as nominative because it is masculine inanimate
   • so the adjective is serijski

So the different endings are not random—they match the grammar of the nouns.

Yes, Croatian word order is more flexible than English, but not completely free.

This sentence has a neutral, natural order:

• Svjedok je blizu nadzorne kamere i zna serijski broj.

You can move things for emphasis, but you must be careful with je, because it is a clitic and usually goes in the second position of its clause.

For example:

• Blizu nadzorne kamere je svjedok. = Near the surveillance camera is the witness.
  This is possible, but more marked.

The original sentence is the safest and most neutral version for learners.

A few useful pronunciation points:

• j in Croatian is pronounced like English y in yes
• je sounds roughly like ye
• svjedok begins with a cluster svj-, roughly svye-
• serijski contains j again, so it is not pronounced like English serial

Very roughly:

• svjedok ≈ SVYE-dok
• je ≈ ye
• serijski ≈ seh-REE-yee-skee

These are only approximations, but the key thing is:

• pronounce j as y
• pronounce each written vowel clearly

Svjedok is grammatically masculine in form.

In many contexts, especially legal or formal ones, svjedok can still be used for a female witness. But Croatian also has the specifically feminine form:

• svjedokinja = female witness

So depending on style and context, you may see either:

• Svjedok je...
• Svjedokinja je...

In your sentence, svjedok is the form that appears, and it is perfectly normal.