Rozbieżność jest niezbyt poważna.

Because rozbieżność is a feminine noun, and the adjective has to agree with it in gender, number, and case.

Here, both words are:

• feminine
• singular
• nominative

So the adjective appears as poważna.

Compare:

• poważny problem — a serious problem
• poważna rozbieżność — a serious discrepancy
• poważne pytanie — a serious question

Yes, rozbieżność is feminine.

A lot of Polish nouns ending in -ość or -ność are feminine, for example:

• możliwość — possibility
• trudność — difficulty
• zgodność — conformity
• rozbieżność — discrepancy/divergence

Even though it does not end in -a, it is still feminine.

It is in the nominative case, because it is the subject of the sentence.

The sentence structure is basically:

• Rozbieżność — subject
• jest — is
• niezbyt poważna — predicate description

So this is the basic dictionary form of the noun.

In standard Polish, jest is normally used in this kind of present-tense sentence.

So:

• Rozbieżność jest niezbyt poważna. — natural and standard

You may sometimes see jest omitted in certain styles, such as notes, headlines, or more compressed speech:

• Rozbieżność niezbyt poważna.

But that sounds less complete and less neutral. For a normal full sentence, keep jest.

Niezbyt means not very, not particularly, or rather not very.

So niezbyt poważna means:

• not very serious
• not particularly serious

It softens the adjective instead of completely negating it.

Compare:

• poważna — serious
• niezbyt poważna — not very serious
• niepoważna — not serious / unserious

Those are not exactly the same.

Because niezbyt functions as a fixed adverb meaning not very.

It is normally written as one word:

• niezbyt trudny
• niezbyt daleko
• niezbyt poważna

A learner may expect something more literally like not too, but Polish uses niezbyt as a standard single-word modifier.

They are similar, but not always identical in tone.

• niezbyt poważna sounds fairly neutral and standard
• nie bardzo poważna can sound a bit more conversational

In this sentence, niezbyt poważna is a very natural choice.

In Polish, adjectives often come before the noun:

• poważna rozbieżność

But in this sentence, poważna is not directly modifying the noun inside a noun phrase. Instead, it is part of the predicate after jest:

• Rozbieżność jest poważna.
• Rozbieżność jest niezbyt poważna.

So in English terms, this is more like:

• subject + is
  • adjective

That is why the adjective appears after the verb.

Yes, Polish word order is more flexible than English, but the neutral order here is:

• Rozbieżność jest niezbyt poważna.

You could rearrange it for emphasis, for example:

• Niezbyt poważna jest ta rozbieżność.
• Ta rozbieżność jest niezbyt poważna.

But these versions change the focus or sound more marked. The original sentence is the most neutral and natural for general use.

A rough pronunciation guide is:

roz-BYEZH-noshch

A few useful points:

• rz usually sounds like the s in measure or zh
• bie sounds roughly like bye
• ż also sounds like zh
• ń is a soft n
• ść at the end is a soft cluster, something like shch or shch-t to English ears

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

• roz-BIEŻ-ność

Yes. It is an abstract noun, and it often appears in more formal or analytical language.

It can refer to:

• a discrepancy
• a divergence
• a mismatch
• a difference between two things that should align

So this is not usually a casual everyday word like problem. It feels a bit more formal or technical depending on context.

Not naturally in this sentence.

Polish sometimes uses to in equational sentences, especially when identifying or defining something:

• To jest problem.
• Warszawa to stolica Polski.

But here you are describing a quality:

• Rozbieżność jest poważna.
• Rozbieżność jest niezbyt poważna.

So jest is the correct choice.

Yes — and that is a very natural alternative.

Compare:

• Rozbieżność jest niezbyt poważna.
• Rozbieżność nie jest zbyt poważna.

Both mean roughly the same thing: The discrepancy is not very serious.

A small nuance:

• jest niezbyt poważna feels slightly more compact
• nie jest zbyt poważna may feel a little more transparent to learners because the negation is attached to the verb

Both are correct and common.