Ubezpieczenie jest ważne.

Because ubezpieczenie is a neuter singular noun in Polish.

Adjectives in Polish must agree with the noun they describe in gender, number, and often case.

So:

• ważny = masculine singular
• ważna = feminine singular
• ważne = neuter singular

Since ubezpieczenie is neuter, the correct form is ważne.

Ubezpieczenie is neuter.

A good clue is its ending: nouns ending in -enie are very often neuter in Polish.

So you get:

• to ubezpieczenie
• ubezpieczenie jest ważne

That neuter gender is exactly why the adjective appears as ważne.

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

The sentence structure is:

• Ubezpieczenie = the subject
• jest = is
• ważne = an adjective functioning as the predicate, describing the subject

After jest, the adjective here also stays in the matching nominative form: ważne.

It can mean either, depending on context.

In Polish, ważny / ważna / ważne can mean:

• important
• valid / in force / current

With things like:

• documents
• passports
• tickets
• insurance

it often means valid.

So in a sentence like Ubezpieczenie jest ważne, the natural meaning is often The insurance is valid or The insurance is in force, not necessarily important.

In normal standard Polish, yes, you should keep it.

Polish usually does not drop the present-tense form of to be in ordinary sentences like this. So:

• Ubezpieczenie jest ważne = correct, normal
• Ubezpieczenie ważne = possible only in very limited contexts, such as notes, labels, headlines, or very elliptical speech

For a learner, the safe rule is:

• use jest in full sentences like this

Because Polish has no articles.

English uses:

• a
• an
• the

Polish does not. So ubezpieczenie can mean things like:

• insurance
• the insurance
• an insurance policy

The exact meaning depends on context.

That is very normal in Polish, and learners have to get used to reading article-like meaning from the situation rather than from a separate word.

Yes. Polish word order is more flexible than English, because endings do a lot of the grammatical work.

The neutral order here is:

• Ubezpieczenie jest ważne.

But you could also hear:

• Ważne jest ubezpieczenie.

That version shifts the emphasis, roughly like It is the insurance that is valid/important or What is valid/important is the insurance.

So the original sentence is the most neutral, everyday way to say it.

A rough pronunciation guide is:

• ubezpieczenie ≈ oo-bez-pye-CHE-nye
• jest ≈ yest
• ważne ≈ VAZH-ne

A few useful sound notes:

• cz sounds like ch in chocolate
• ż sounds like the s in measure
• nie at the end sounds roughly like nye
• Polish stress is usually on the second-to-last syllable, so:
  • ubezpieczenie
  • ważne

More carefully, the stress falls like this:

• u-bez-pie-CZE-nie
• WAŻ-ne

Jest is the 3rd person singular present tense of the verb być, meaning to be.

So:

• jest = is

Examples:

• On jest zmęczony. = He is tired.
• To jest dobre. = This is good.
• Ubezpieczenie jest ważne. = The insurance is valid / important.

It matches the singular subject ubezpieczenie.

It is singular.

The plural is ubezpieczenia.

So:

• ubezpieczenie jest ważne = the insurance / insurance policy is valid
• ubezpieczenia są ważne = the insurance policies are valid

Notice that in the plural, both the verb and the adjective change:

• jest → są
• ważne stays ważne here because that is also a plural non-masculine-personal form

Yes, very often it can.

Depending on context, ubezpieczenie may refer to:

• insurance in general
• insurance coverage
• a specific insurance policy

So Ubezpieczenie jest ważne could mean something like:

• The insurance is valid
• The policy is valid
• The coverage is in force

Context tells you which one is meant.