Это несомненно важно.

Because важно is the form Russian commonly uses in statements like Это важно = This is important / It is important.

More specifically, важно is the short-form neuter predicate form of важный. In sentences like this, Russian often uses this predicative form after это.

Compare:

• важный = important, masculine full adjective
  
  • важный вопрос = an important question
• важно = important, in the sense of is important
  
  • Это важно = This is important

So in this sentence, важно is the natural form because it is part of the predicate, not directly describing a noun.

Yes—sort of.

In the present tense, Russian usually omits the verb to be. So:

• Это важно literally looks like This important
• but it means This is important

You can think of an invisible is here. Russian does this very often:

• Он дома = He is at home
• Она занята = She is busy
• Это важно = This is important

So the full idea is there, even though no separate word for is appears.

Это usually means this, but depending on context it can also feel like it or that in English.

In Это несомненно важно, это refers to some fact, action, idea, or situation already known from context:

• Это важно = This is important
• Это трудно = This is difficult
• Это понятно = This is clear

So это is a very common way to point to something in a general way.

Here, несомненно functions like an adverb meaning undoubtedly, certainly, or without doubt.

It modifies the whole statement or the predicate важно:

• Это важно = This is important
• Это несомненно важно = This is undoubtedly important

A useful thing to know is that несомненно can also behave like a parenthetical word meaning of course / undoubtedly, which affects punctuation. That leads to the next common question.

Because in this sentence, несомненно can be understood as directly modifying важно: undoubtedly important.

So:

• Это несомненно важно. = This is undoubtedly important.

But you may also see:

• Это, несомненно, важно.

With commas, несомненно is treated more like a parenthetical comment: This, undoubtedly, is important.

Both are possible, but the version without commas is very natural when несомненно is closely tied to важно.

A simple rule:

• No commas: when it works like an ordinary adverb
• Commas: when it feels like an inserted comment by the speaker

Yes. Russian word order is flexible, though different orders can sound slightly different in emphasis.

For example:

• Это несомненно важно.
  neutral, natural
• Это важно, несомненно.
  puts важно first, then adds undoubtedly
• Несомненно, это важно.
  starts with undoubtedly, which gives it more emphasis

All of these can be correct, but the original sentence is a very standard, neutral way to say it.

The stress is:

• Э́то
• несомне́нно
• ва́жно

So: Э́то несомне́нно ва́жно.

A rough pronunciation guide:

• Это ≈ EH-tuh
• несомненно ≈ nee-sam-NYEN-nuh
• важно ≈ VAZH-nuh

A couple of pronunciation notes:

• о often sounds closer to a when unstressed
• ч is not involved here, but ж in важно is like the s in measure
• что is not in this sentence, so nothing tricky there

This is a very common source of confusion.

Formally, важно comes from the adjective важный, but in a sentence like Это важно, it is best understood as a predicative form—a word used to state a condition or quality: is important.

It may look like an adverb because many adverbs in Russian also end in -о, but here it is not really the same as the adverb in a phrase like:

• Он важно говорит = He speaks importantly / pompously

In Это важно, важно is describing the situation expressed by the whole sentence, not describing how someone does something.

Yes.

• Это очень важно. = This is very important.
• Это несомненно важно. = This is undoubtedly important.

The difference is in what kind of emphasis you want:

• очень strengthens the degree: very
• несомненно strengthens the certainty: undoubtedly

So:

• очень важно = high importance
• несомненно важно = no doubt that it is important

You can even combine them:

• Это несомненно очень важно. = This is undoubtedly very important.

Because it comes from несомненный.

Very roughly:

• сомнение = doubt
• несомненный = unquestionable, beyond doubt
• несомненно = unquestionably, undoubtedly

When the adverb is formed from несомненный, the double н stays:

• несомненный
• несомненно

This is mainly a spelling point, but it is a common one for learners.

It works in both everyday and slightly formal Russian.

• In normal conversation, it sounds natural.
• In writing, argumentation, or presentations, it also sounds good.
• несомненно is a bit more formal or polished than something like конечно in some contexts, but it is still very standard.

So Это несомненно важно. is a perfectly normal and useful sentence to know.