Rozbieżność jest poważna.

In Polish, not all feminine nouns end in -a. A lot of feminine nouns end in a soft consonant, and nouns ending in -ość are very commonly feminine.

So rozbieżność is a feminine singular noun, and you can see that from agreement:

• ta rozbieżność = this discrepancy
• poważna rozbieżność = a serious discrepancy

The adjective takes the feminine form, which confirms the noun’s gender.

It is in the nominative singular.

That is because rozbieżność is the subject of the sentence:

• Rozbieżność = the thing being talked about
• jest poważna = is serious

So the structure is basically:

• [subject in nominative] + jest + [adjective agreeing with the subject]

Because the adjective has to agree with rozbieżność in gender, number, and case.

Here, rozbieżność is:

• feminine
• singular
• nominative

So the adjective must also be feminine singular nominative:

• masculine: poważny
• feminine: poważna
• neuter: poważne

The dictionary form of the adjective is usually poważny, but in this sentence the correct form is poważna.

Because this is a predicate adjective after jest.

In Polish, when an adjective comes after być (to be) and describes the subject, it normally stays in the nominative:

• Sprawa jest ważna. = The matter is important.
• Rozbieżność jest poważna. = The discrepancy is serious.

So poważna is correct.

A form like poważną would be the instrumental or accusative form, and it does not fit this sentence.

Jest is the 3rd person singular present form of być (to be), so here it means is.

• rozbieżność = discrepancy
• jest = is
• poważna = serious

So the sentence literally follows the pattern:

• Discrepancy is serious.

Can it be omitted? In standard neutral Polish, learners should keep jest here. If you remove it:

• Rozbieżność poważna

that sounds elliptical, headline-like, or very context-dependent. It is not the normal full sentence for a beginner to copy.

Because Polish does not have articles.

So rozbieżność can mean:

• a discrepancy
• the discrepancy

Which one is intended depends on context.

In this sentence, English might translate it as either:

• The discrepancy is serious.
• A discrepancy is serious.

But in real use, the context usually makes the intended meaning clear.

No, Polish word order is fairly flexible, but the neutral order here is:

• Rozbieżność jest poważna.

That is the most straightforward way to say it.

Other orders are possible, but they change the emphasis:

• Poważna jest rozbieżność.
  This is marked and sounds contrastive or stylistically special.

• Jest poważna rozbieżność.
  This often shifts toward the meaning There is a serious discrepancy rather than The discrepancy is serious.

So for normal use, Rozbieżność jest poważna is the safest pattern.

The stress is on the second-to-last syllable:

• roz-BIEŻ-ność

A rough English-friendly approximation is:

• roz-BYEZH-noshch

More precisely, in IPA it is:

• /rɔzˈbjɛʐ.nɔɕt͡ɕ/

A few useful notes:

• rz here sounds like ż
• ż is somewhat like the s in measure, but more retroflex
• ść is a soft sound, roughly like a very soft shch

If you want to sound natural, getting the stress right is the first priority.

You put nie before the verb:

• Rozbieżność nie jest poważna. = The discrepancy is not serious.

This is the normal Polish pattern for negating to be:

• jest = is
• nie jest = is not

The plural would be:

• Rozbieżności są poważne. = The discrepancies are serious.

Notice how everything changes to match the plural subject:

• singular: rozbieżność jest poważna
• plural: rozbieżności są poważne

So both the verb and the adjective agree with the noun.