Komunikacijske vještine su potrebne.

Because it is an adjective agreeing with vještine.

• vještine = skills
• It is feminine plural nominative here
• So the adjective must match it in gender, number, and case

That is why you get:

• komunikacijska vještina = a communication skill
• komunikacijske vještine = communication skills

So komunikacijske is simply the form that matches vještine.

It is plural here.

The basic singular form is:

• vještina = skill

The plural is:

• vještine = skills

In this sentence, vještine is the subject, so it is in the nominative plural.

Su means are. It is the 3rd person plural present form of biti (to be).

So:

• vještine su potrebne = skills are necessary / skills are needed

Since the subject is plural, the verb must also be plural:

• je = is
• su = are

Because vještine is plural, we use su.

Because potrebne must agree with vještine.

Here potreban / potrebna / potrebno works like an adjective meaning necessary / needed. In this sentence it is in the feminine plural form:

• potreban = masculine singular
• potrebna = feminine singular
• potrebno = neuter singular
• potrebni = masculine plural
• potrebne = feminine plural

Since vještine is feminine plural, the correct form is potrebne.

It is in the nominative plural.

Why?

Because it is the subject of the sentence:

• Komunikacijske vještine = communication skills
• su potrebne = are necessary

Subjects are normally in the nominative, so both the noun and its adjective are in the nominative plural here.

Yes. Croatian word order is more flexible than English word order.

This sentence can also be said as:

• Potrebne su komunikacijske vještine.

That version is often very natural too, and it puts slightly more focus on potrebne.

The original sentence:

• Komunikacijske vještine su potrebne.

is also correct and sounds neutral, with a little more focus on komunikacijske vještine as the topic.

So both are fine.

Normally, in standard Croatian, no. You usually keep the present-tense form of to be in this kind of sentence.

So the normal sentence is:

• Komunikacijske vještine su potrebne.

Leaving out su would sound incomplete in ordinary standard Croatian.

It is a very good and natural way to say it, but not the only possible expression.

A learner may also see:

• komunikacijske vještine
• komunikativne vještine

These are close, but not always exactly the same.

• komunikacijske = related to communication
• komunikativne = communicative, good at interacting/talking with people

In many contexts, both can work, but komunikacijske vještine is a very standard translation for communication skills.

Approximately: VYESH-tee-neh

A few useful points:

• vj is pronounced together, starting with a v sound followed by ye
• ješ sounds roughly like yesh
• št is like sht
• stress in Croatian is not always marked in normal writing, so learners often first focus on getting the sounds right

So a rough English-friendly guide is:

• vještine ≈ VYESH-tee-neh

Because Croatian does not use articles like English a and the.

So:

• vještina can mean a skill or the skill, depending on context
• vještine can mean skills or the skills

The exact meaning is understood from context rather than from articles.

Yes. Croatian often uses trebati for the idea of to need / to be needed.

For example, you could say:

• Potrebne su komunikacijske vještine.
• Komunikacijske vještine su potrebne.

These mean that communication skills are necessary.

In other contexts, Croatian might also say something like:

• Trebaju vam komunikacijske vještine. = You need communication skills.

So the sentence you have uses an adjective structure (are necessary), but Croatian can also express the idea with trebati.