Разница важна.

In Russian, the verb to be is usually omitted in the present tense.

So:

• Разница важна = The difference is important
• literally: Difference important

This is completely normal Russian. You do not need a present-tense word for is here.

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

Its dictionary form is:

• разница = difference

In this sentence, разница is the thing being described as important.

Важна is the short form of the adjective важный / важная / важное / важные.

Russian often uses short-form adjectives when saying that something is a certain way, especially in more formal or neutral statements.

Here:

• разница is feminine singular
• so the short-form adjective must also be feminine singular
• therefore: важна

Compare:

• важный = masculine full form
• важная = feminine full form
• важна = feminine short form

So Разница важна is grammatically matched.

Both come from the adjective important, but they are used differently.

• важная is the full form
• важна is the short form

Very roughly:

• важная разница = an important difference
  (the adjective directly modifies the noun)
• Разница важна = The difference is important
  (the adjective is the predicate)

So in this sentence, важна is the natural choice because it means is important.

A practical pronunciation guide:

• ра́зница ≈ RAZ-nee-tsa
• важна́ ≈ vazh-NA

Stress:

• ра́зница — stress on the first syllable
• важна́ — stress on the last syllable

A smooth pronunciation would sound something like:

• RAZ-nee-tsa vazh-NA

Yes, Russian word order is more flexible than English.

The neutral order is:

• Разница важна.

But you may also see:

• Важна разница.

That can sound more marked or context-dependent, with extra emphasis on important.

For a learner, Разница важна is the safest and most neutral version.

Usually, yes, but it can also mean distinction or the difference between things, depending on context.

For example:

• Есть разница. = There is a difference.
• В чём разница? = What is the difference?

So in Разница важна, it most naturally means The difference is important.

Russian has no articles like a or the.

So разница can mean:

• a difference
• the difference

The exact meaning depends on context.

In a standalone sentence like Разница важна, English will often translate it as The difference is important, but Russian itself does not explicitly mark the.

It sounds neutral and perfectly natural. Because it uses a short-form adjective (важна), it can feel slightly more concise or formal than some English equivalents, but it is not unnatural or overly bookish.

It works well in both speech and writing.

You technically can, but it is much less natural in ordinary speech.

• Разница важна = natural, simple, normal
• Разница является важной = more formal, heavier, often bureaucratic or academic in tone

Also notice that with является, the adjective would normally appear in a different form:

• важной (instrumental), not важна

So for everyday Russian, Разница важна is the better choice.