Você tem certeza?

Because você takes third-person singular verb forms in Portuguese.

So:

• você tem = you have
• ele tem = he has
• ela tem = she has

This is normal in Portuguese. If you used tu, the standard form would be tu tens certeza?

In most of Brazil, você is the normal, neutral everyday way to say you to one person.

A few useful comparisons:

• você = common, neutral
• o senhor / a senhora = more formal, respectful
• tu = used in some regions and social groups

So Você tem certeza? sounds very natural in Brazilian Portuguese.

Yes. That is very common.

Portuguese often omits subject pronouns when the meaning is clear from context. So both are natural:

• Você tem certeza?
• Tem certeza?

Including você can add a little emphasis, clarity, or contrast.

Because ter certeza is the normal idiomatic expression in Portuguese.

Literally, it is:

• ter = to have
• certeza = certainty

So Portuguese expresses this idea as to have certainty. English uses to be sure, but Portuguese usually does not copy that structure here.

A related expression is estar certo, but it is not always the same in tone or usage. If you want a direct, common way to say Are you sure?, Você tem certeza? is the safest choice.

A simple approximate pronunciation is:

• você → voh-SEH
• tem → teyn or tehng depending on accent
• certeza → sehr-TEH-zah

A few notes:

• The stress in você is on the last syllable: vo-SE
• In certeza, the c sounds like s
• The z sounds like the z in zero
• The final a in certeza is usually weak in Brazilian Portuguese

A more natural connected rhythm is something like: voh-SEH teyn sehr-TEH-zah?

The accent mark in você shows that the stress falls on the last syllable and that the vowel is pronounced as ê, not é.

So:

• vo-CÊ

The accent is part of the correct spelling. It is not optional.

Because ter certeza is a fixed, very common expression.

So you normally say:

• tenho certeza
• tem certeza
• temos certeza

You can sometimes hear a certeza in other structures, such as:

• Tenho a certeza de que...

But in everyday Brazilian Portuguese, ter certeza without an article is extremely common and natural.

Very often, people shorten it.

Common spoken versions include:

• Tem certeza?
• Cê tem certeza?
• Tem certeza mesmo?

Notes:

• cê is a reduced spoken form of você
• mesmo adds emphasis, like really

So in casual speech, you may hear Cê tem certeza? a lot.

Some natural answers are:

• Sim. = Yes.
• Tenho certeza. = I am sure.
• Tenho, sim. = Yes, I am.
• Não tenho certeza. = I am not sure.
• Acho que sim. = I think so.
• Não sei. = I do not know.

A very direct textbook-style answer is Sim, tenho, but in real conversation, Tenho certeza or just Sim is often more natural.

Yes, depending on tone.

It can be:

• a neutral request for confirmation
• a caring question
• a skeptical challenge

For example:

• gentle tone: just checking
• sharp tone: doubting the person
• stressed mesmo: stronger challenge

So the phrase itself is not rude, but intonation matters a lot.

A very common way is:

• Você tem certeza disso?
• Tem certeza disso?

Here, disso means about that.

If what follows is a whole clause, you can also use:

• Você tem certeza de que...?

Example pattern:

• Você tem certeza de que ele vem? = Are you sure that he is coming?

The standard form would be:

• Tu tens certeza?

However, in some parts of Brazil, people mix tu with the verb form tem in casual speech:

• Tu tem certeza?

That mixed form is common in some regions, but if you are learning standard grammar, tu tens certeza? is the form to remember.