Ova kvalifikacija je važna.

• ova = this
• kvalifikacija = qualification
• je = is
• važna = important

So the full sentence means This qualification is important.

Because kvalifikacija is a feminine singular noun, and the demonstrative ovaj / ova / ovo must agree with it.

The forms are:

• ovaj = masculine singular
• ova = feminine singular
• ovo = neuter singular

Since kvalifikacija is feminine, you use ova.

For the same reason: agreement.

The adjective važan means important, but its form changes depending on the gender and number of the noun it describes.

Here are the singular forms:

• važan = masculine
• važna = feminine
• važno = neuter

Because kvalifikacija is feminine singular, the correct form is važna.

It is in the nominative singular.

That is because kvalifikacija is the subject of the sentence: This qualification is important.

In simple statements like this, the subject is normally in the nominative.

In this sentence, je is the 3rd person singular present form of biti = to be.

Croatian normally does use the verb to be in sentences like this:

• Ova kvalifikacija je važna. = This qualification is important.

So unlike in some languages, you generally do not leave out je here in standard Croatian.

This is the most neutral and natural word order for a basic statement:

• subject: ova kvalifikacija
• verb: je
• predicate adjective: važna

Croatian word order is more flexible than English, so other orders are possible, for example:

• Važna je ova kvalifikacija.
• Ova je kvalifikacija važna.

But those may sound more marked, emphatic, or context-dependent. The original order is the safest default.

Yes, kvalifikacija is a feminine noun.

A lot of Croatian nouns ending in -a are feminine, though not absolutely all. In this case, kvalifikacija is definitely feminine, which is why both ova and važna are feminine too.

A rough English-friendly guide is:

OH-vah kva-lee-fee-kah-TSI-yah yeh VAZH-nah

A few useful notes:

• j is pronounced like English y
• ž sounds like the s in measure
• cija in kvalifikacija is pronounced roughly tsi-ya

So:

• ova ≈ OH-vah
• kvalifikacija ≈ kva-lee-fee-kah-TSI-yah
• je ≈ yeh
• važna ≈ VAZH-nah

Yes, but the meaning changes slightly.

• ova kvalifikacija = this qualification
• ta kvalifikacija = that qualification or sometimes that qualification you mentioned / that specific qualification
• ona kvalifikacija = that qualification over there / that one

So if you specifically mean this qualification, ova is the correct choice.

Because in this sentence važna is part of the predicate:
This qualification is important.

So the structure is:

• Ova kvalifikacija = the subject
• je važna = is important

If you put the adjective before the noun, you get a different structure:

• važna kvalifikacija = an important qualification

So:

• Ova kvalifikacija je važna. = This qualification is important.
• Ova važna kvalifikacija... = This important qualification...

Yes.

• Kvalifikacija je važna. = The qualification is important or A qualification is important, depending on context
• Ova kvalifikacija je važna. = This qualification is important

Adding ova makes the noun more specific and points to a particular qualification.

Yes. Je is a clitic, which means it tends to appear in a special early position in the sentence, often called second position.

That is why Croatian often prefers patterns like:

• Ova kvalifikacija je važna.
• Ova je kvalifikacija važna.

You usually do not place je completely freely. Learners do not need to master all clitic rules immediately, but it is good to know that short forms like je have placement rules.

The demonstrative and adjective would change to match the noun.

For example:

• masculine: Ovaj dokument je važan.
  = This document is important.

• neuter: Ovo pravilo je važno.
  = This rule is important.

Compare with your sentence:

• feminine: Ova kvalifikacija je važna.

This is a very important Croatian pattern: words that go with the noun must agree in gender, number, and often case.