Você sabe quão importante isso é?

Quão means how in the sense of to what degree or how much.

So quão importante = how important.

It is used to intensify an adjective or adverb:

• quão importante = how important
• quão difícil = how difficult
• quão cedo = how early

In this sentence, it introduces the idea of degree: how important this is.

Not very common in everyday spoken Brazilian Portuguese. Native speakers will understand it, but it sounds more formal, literary, or rhetorical than normal conversation.

In everyday Brazilian Portuguese, people are more likely to say:

• Você sabe o quanto isso é importante?
• Você sabe como isso é importante? (less standard in some contexts, but common in speech)
• Você sabe o quanto isso importa?

So quão is correct, but it may sound elevated or written.

The order quão importante isso é is the most natural and standard way to build this kind of embedded clause.

It mirrors the structure:

• how important this is

In Portuguese, this type of clause often keeps the subject before the verb:

• quão importante isso é
• o quanto isso é importante

You may sometimes hear quão importante é isso, but it is less neutral here and can sound more marked or stylistic. For a learner, quão importante isso é is the safer model.

Because the part quão importante isso é works like an embedded clause: how important this is.

Compare:

• Isso é importante. = This is important.
• Você sabe quão importante isso é? = Do you know how important this is?

Portuguese does not need the same kind of word order change English sometimes uses in direct questions. Inside a clause like this, it is normal for the sentence to end with the verb é.

Yes. O quão is also used, and many learners will encounter both:

• Você sabe quão importante isso é?
• Você sabe o quão importante isso é?

Both are understandable and correct. In practice, o quão is often felt as a little more natural than bare quão, especially in modern usage, though both still sound somewhat formal.

In very natural everyday Brazilian Portuguese, o quanto is usually more common:

• Você sabe o quanto isso é importante?

Você means you and is the subject of sabe.

Yes, it can be omitted:

• Sabe quão importante isso é?

Brazilian Portuguese often allows subject pronouns to be dropped when the meaning is clear. However, because você uses third-person verb forms, keeping você can help avoid ambiguity and often sounds clearer.

So:

• Você sabe... = explicit, clear
• Sabe... = also possible, especially in conversation

Because saber is the verb used for knowing a fact, knowing information, or being aware of something.

Here the idea is:

• Do you know / are you aware how important this is?

Conhecer is used more for:

• knowing a person
• being familiar with a place
• being acquainted with something

Examples:

• Você sabe a resposta. = You know the answer.
• Você conhece o professor. = You know the teacher.

So sabe is the correct choice here.

Isso means that or this, depending on context in English. In real Brazilian Portuguese, isso is extremely common and often replaces distinctions that grammar books describe more strictly.

Traditional grammar says:

• isto = this, near the speaker
• isso = that, near the listener / something just mentioned
• aquilo = that over there

But in everyday Brazilian Portuguese, isto is much less common in speech, and isso is often used where English might say either this or that.

So isso here is totally normal.

Yes, it is a real question. Portuguese does not usually form questions by inverting the subject and auxiliary the way English does.

English:

• Do you know how important this is?

Portuguese:

• Você sabe quão importante isso é?

The sentence keeps normal declarative word order, and the question is shown by:

• intonation in speech
• the question mark in writing

This is very common in Portuguese.

A very natural everyday version would be:

• Você sabe o quanto isso é importante?

Other common possibilities:

• Você sabe como isso é importante?
• Você tem ideia de como isso é importante?
• Você sabe o quanto isso importa?

Your original sentence is correct, but these alternatives usually sound more conversational in Brazil.

Not exactly.

• quão importante = how important
• tão importante = so important / as important

Examples:

• Você sabe quão importante isso é? = Do you know how important this is?
• Isso é tão importante. = This is so important.

So quão asks about degree, while tão states degree.

In Brazilian Portuguese, quão is pronounced roughly like kwowng, but with a nasal ending. The exact English approximation is imperfect.

A few points:

• qu here sounds like kw
• ão is a nasal diphthong, like the ending in não, pão, então

If you already know não, then quão has a similar nasal ending:

• quão
• não

The nasal sound is important; it should not sound like a plain ow.

Yes, but it depends on region and style.

With tu, the sentence would be:

• Tu sabes quão importante isso é?

In many parts of Brazil, você is much more common. In some regions, people do use tu, but the verb agreement can vary in actual speech:

• Tu sabes... (traditional/standard agreement)
• Tu sabe... (common in some regions in informal speech)

If you are learning general Brazilian Portuguese, você sabe... is the safest form.

It depends mostly on intonation, but the wording with quão makes it sound a bit more formal, emphatic, or rhetorical than neutral everyday speech.

It can sound like:

• serious
• expressive
• written
• dramatic

If you want a more neutral spoken tone, Você sabe o quanto isso é importante? is usually better.

So the original is correct, but not the most casual phrasing.