Završetak radova je važan.

Because završetak is a noun meaning completion / finishing, and nouns like this commonly take a genitive complement in Croatian.

So:

• završetak = completion
• radovi = works, construction works, operations, jobs
• radova = of the works / of the construction work in genitive plural

This is similar to how English says the completion of the works.

The basic pattern is:

• završetak nečega = completion of something

Examples:

• završetak projekta = completion of the project
• završetak škole = finishing school
• završetak radova = completion of the works

In Croatian, radovi is very often used in the plural when talking about construction works, building works, road works, or similar practical/technical work.

So radovi does not always mean separate countable jobs in the everyday English sense. It is often the normal Croatian way to refer to a body of work being carried out.

That means:

• rad = work, labor, a piece of work
• radovi = works, especially construction/technical works

So završetak radova is a very natural phrase for things like:

• completion of construction works
• completion of road works
• completion of renovation works

Because završetak is a masculine singular noun, and the adjective has to agree with it.

Agreement in Croatian means the adjective matches the noun in:

• gender
• number
• case

Here:

• završetak = masculine singular nominative
• so the adjective must also be masculine singular nominative
• therefore: važan

Compare:

• završetak je važan = the completion is important
• gradnja je važna = the construction is important
• pitanje je važno = the question is important

So:

• masculine: važan
• feminine: važna
• neuter: važno

The subject is Završetak radova.

More precisely:

• the head noun is završetak
• radova depends on it and specifies completion of what?

So the whole noun phrase završetak radova functions as the subject, and je važan is the predicate.

Structure:

• Završetak radova = subject
• je = is
• važan = important

Croatian has a strong tendency for certain short unstressed words, called clitics, to appear in the second position of the clause.

The verb je here is a clitic form of biti (to be), so it usually comes after the first phrase or element.

That is why:

• Završetak radova je važan.

sounds natural.

You will often see this with other clitics too:

• On je ovdje.
• To mi se sviđa.
• Projekt je završen.

This does not mean it must always be the second written word. It is more accurate to think of it as the second slot in the sentence, often after the first phrase.

Yes, Croatian word order is fairly flexible, but not every version sounds equally neutral.

The most neutral version here is:

• Završetak radova je važan.

Other possible orders can shift emphasis:

• Važan je završetak radova.
  This emphasizes important.

• Je važan završetak radova.
  This is generally not a normal neutral sentence opening.

• Završetak je radova važan.
  This sounds unnatural.

So although Croatian allows flexibility, the original sentence is the standard, natural form.

Not exactly.

Both can relate to end, but they are used differently.

• kraj = end, the end point, conclusion
• završetak = completion, finishing, successful bringing to an end

In many contexts, završetak sounds more formal or process-oriented.

Examples:

• kraj filma = the end of the film
• završetak projekta = completion of the project

For radovi, završetak radova is especially natural because it refers to work being finished or completed.

It is in the nominative singular.

You can tell because:

• it is the main noun in the subject
• the adjective važan agrees with it in nominative singular masculine

So:

• završetak = nominative singular
• radova = genitive plural

This is a very common pattern in Croatian:

• nominative head noun + genitive dependent noun

For example:

• početak sastanka = beginning of the meeting
• kraj dana = end of the day
• završetak radova = completion of the works

A helpful approximate pronunciation for an English speaker is:

• Za-vr-she-tak ra-do-va ye va-zhan

A few sound notes:

• ž sounds like the s in measure
• š sounds like sh
• j sounds like English y
• r in Croatian is trilled or tapped, not the English r

So:

• završetak ≈ za-vr-SHE-tak
• je ≈ ye
• važan ≈ VA-zhan

Yes, that is possible, but it means something slightly different in style and structure.

• završetak radova = the completion of the works
• završavanje radova = the process of finishing the works

Also, završavanje is a verbal noun in -anje, and it is neuter, so the adjective becomes važno.

Compare:

• Završetak radova je važan. = The completion of the works is important.
• Završavanje radova je važno. = Finishing the works is important / The process of finishing the works is important.

The first one often sounds more concrete and result-focused. The second sounds more process-focused.

Yes, it sounds fairly neutral-to-formal, and it would fit well in:

• news reports
• official announcements
• project updates
• construction or administrative contexts

That is especially because of the phrase radovi and the noun završetak.

In casual everyday speech, people might still say it, but depending on context they could also use more conversational wording. Still, the sentence itself is completely natural and correct.