Это столь важно.

In this sentence, это most naturally means this or it, depending on context.

• Это важно. = This is important.
• It can also feel like It is important if English uses it more naturally.

Russian это is very flexible. It often points to:

• a thing just mentioned,
• a situation,
• an idea,
• or something physically present.

So the exact English choice depends on context, not just the word itself.

Столь means so, so much, or to such a degree.

So:

• Это столь важно. = This is so important.
• more literally: This is important to such a degree.

A key point: столь is not the most common everyday word for so in modern spoken Russian. It sounds more formal, literary, or emphatic than так.

Both can mean so, but they differ in style.

• Это так важно. = more common, neutral, conversational
• Это столь важно. = more formal, elevated, literary, sometimes slightly dramatic

So столь often appears in:

• writing,
• speeches,
• formal statements,
• emotional or rhetorical language.

A native speaker would usually say Это так важно in ordinary conversation.

They are similar, but not always identical in tone or use.

• столь = so, to such a degree; more literary/formal
• настолько = so, to such an extent; common and often used when the degree is being emphasized clearly

Compare:

• Это столь важно.
• Это настолько важно.

Both are correct. Настолько often feels a bit more explicit and can strongly suggest a continuation like:

• Это настолько важно, что нельзя откладывать.
  It is so important that it cannot be postponed.

Столь is often stylistically lighter but more bookish.

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

So:

• Это важно. literally looks like:
• This important.

But it means:

• This is important.

This is completely normal in Russian. In the present tense, you usually do not say a separate word for is, am, or are.

In past or future tense, forms of to be do appear:

• Это было важно. = This was important.
• Это будет важно. = This will be important.

Because важно here is not just the regular adjective agreeing with a noun. It is being used in a predicative way, meaning important as a state or quality of the whole situation.

Compare:

• важный = full adjective form, used before a noun

  • важный вопрос = an important question

• важно = predicative form meaning important

  • Это важно. = This is important.

This is very common in Russian:

• Это трудно. = This is difficult.
• Это полезно. = This is useful.
• Это интересно. = This is interesting.

So after это, Russian often uses this -о form.

This is a great question, because it can look like an adverb.

Formally, важно can function as:

• the neuter short-form/predicative form related to важный, or
• an adverb in other contexts.

In Это столь важно, it is best understood as a predicative word meaning important.

Compare:

• Он говорит важно. = He speaks importantly / in an important manner.
  Here важно behaves more like an adverb.

But:

• Это важно. = This is important.
  Here it expresses a state/quality, not the manner of an action.

So for learners, the safest practical idea is:

• in Это важно, translate it as important
• do not think of it as importantly here

Yes, but the neutral order here is:

• Это столь важно.

Russian word order is flexible, and changing it affects emphasis.

For example:

• Это столь важно. = neutral; This is so important.
• Столь важно это. = marked, poetic, or strongly emphatic
• Важно это. = possible in special contexts, but not the default

For most learners, Это столь важно is the safest and most natural order in this exact sentence.

Yes, it can stand alone as a complete sentence.

However, words like столь often introduce a stronger sense of degree, so native speakers may expect a continuation such as:

• Это столь важно, что нельзя ошибиться.
  This is so important that one cannot make a mistake.

Still, Это столь важно. by itself is perfectly grammatical. It just sounds a little formal or rhetorically strong.

A natural stress pattern is:

• Э́то столь ва́жно.

Stress:

• э́то
• столь is one syllable
• ва́жно

Rough pronunciation:

• EH-tuh stol VAZH-nuh

Notes:

• о in unstressed syllables is reduced in normal speech
• столь has a clear st cluster at the beginning

So it sounds approximately like:

• EH-tuh stol VAHZH-nuh

Yes, but not as often as Это так важно.

A native speaker might choose Это столь важно when they want:

• a formal tone,
• a literary tone,
• extra emotional weight,
• or a slightly elevated style.

In everyday speech, Это так важно is more common.

So:

• Это столь важно = correct, natural in the right style
• Это так важно = more typical in ordinary conversation