Это крайне важно.

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

So:

• Это крайне важно = This is extremely important
• literally: This extremely important

You do use forms of быть in the past and future:

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

But in the present tense, Russian normally leaves is/am/are unstated.

Это most often means this, that, or it, depending on context.

In this sentence, это is the subject-like element:

• Это крайне важно = This/That/It is extremely important

A useful thing to know is that это does not change for gender here. Even if the thing being referred to is masculine or feminine, Russian often still uses это in this kind of sentence.

For example:

• Экзамен? Это важно. = The exam? It’s important.
• Идея? Это важно. = The idea? It’s important.

So here, это is functioning a bit like this/that/it in English.

Крайне means extremely, highly, or utterly. It is stronger and often a bit more formal than очень.

Compare:

• Это очень важно. = This is very important.
• Это крайне важно. = This is extremely important.

So крайне adds stronger emphasis. You will often see it in formal speech, writing, warnings, or serious statements.

Важно is the short-form neuter version of the adjective важный (important).

Russian often uses short-form adjectives in sentences like:

• Это важно. = This is important.
• Это понятно. = This is understandable.
• Это возможно. = This is possible.

The long form важный is used when the adjective directly describes a noun:

• важный вопрос = an important question
• важный человек = an important person

But when the idea is more like it is important, Russian usually prefers the short form:

• Это важно

So in this sentence, важно is the natural predicate form.

It may look similar to an adverb, but here it is not functioning as a normal adverb like quickly or well.

In Это крайне важно, важно is a short-form adjective used predicatively, meaning something like is important.

Compare:

• Он говорит важно. = He speaks in an important/solemn manner or pretentiously
  Here важно acts more like an adverb.
• Это важно. = This is important
  Here it is the predicate adjective.

So the same form can appear in different roles, but in your sentence it means important, not importantly.

Because крайне modifies важно, just like extremely modifies important in English.

So:

• крайне важно = extremely important

This is the normal order. Russian adverbs usually come before the word they modify:

• очень хорошо = very well
• совсем неясно = completely unclear
• крайне важно = extremely important

Yes, Russian word order is flexible, but the neutral, standard order here is:

• Это крайне важно.

Other orders are possible for emphasis, for example:

• Крайне важно это.
• Важно это крайне.
  (this one is much less natural in ordinary speech)

In normal conversation and writing, Это крайне важно is the most straightforward and natural version.

Here это is in the nominative case, because it functions as the subject-like element of the sentence.

In simple equational or descriptive sentences like this, the thing being discussed is typically nominative:

• Это хорошо.
• Это странно.
• Это важно.

So you do not need accusative or another case here.

A rough pronunciation guide is:

• Это = EH-tuh
• крайне = KRAI-nye
• важно = VAZH-nuh

Stress:

• Э́то
• кра́йне
• ва́жно

A rough full pronunciation:

• EH-tuh KRAI-nye VAZH-nuh

A common difficulty for English speakers is ж in важно, which sounds like the s in measure.

Yes, absolutely. That is probably the more common everyday version.

• Это очень важно = This is very important
• Это крайне важно = This is extremely important

Use очень for a normal, common very.
Use крайне when you want stronger emphasis or a slightly more formal tone.

It is mostly neutral to somewhat formal, depending on context.

• Это важно sounds neutral.
• Это очень важно sounds neutral and common.
• Это крайне важно sounds stronger, more emphatic, and often slightly more formal.

You might hear крайне in:

• official statements
• instructions
• serious discussions
• written language

So the sentence is not strange at all, but it carries more weight than the version with очень.