Tenho certeza.

Because ter certeza is the normal Portuguese expression for to be sure or to be certain.

Portuguese often uses ter + noun where English uses to be + adjective. So:

• Tenho certeza = literally I have certainty
• natural English = I’m sure

So sou certeza or estou certeza would not work.

A different correct option is Estou certo / Estou certa, but that is a different structure.

Tenho is the 1st person singular present tense form of the verb ter.

So it means:

• I have
• and in this expression, I am sure

A few present-tense forms of ter are:

• eu tenho = I have
• você tem = you have
• ele/ela tem = he/she has
• nós temos = we have
• eles/elas têm = they have

Notice that tenho is irregular, so it does not follow a perfectly regular pattern.

Portuguese often drops subject pronouns when the verb already makes the subject clear.

Since tenho clearly means I have, the eu is optional:

• Tenho certeza.
• Eu tenho certeza.

Both are correct.

Adding eu can give extra emphasis, contrast, or clarity, but in everyday speech it is very common to leave it out.

Certeza is a noun, not an adjective.

That matters because the sentence is built as:

• tenho = I have
• certeza = certainty

So the structure is literally I have certainty, not I am certain.

This is why learners should memorize ter certeza as a set expression.

In the basic expression ter certeza, Portuguese usually does not use an article.

So the normal form is:

• Tenho certeza.

Not usually:

• Tenho a certeza.

However, a certeza can appear in more specific contexts, especially when referring to a particular certainty:

• Tenho a certeza de que ele vem.

In Brazil, though, tenho certeza de que... without the article is also extremely common and often more natural in everyday speech.

So for learners, the safest basic pattern is:

• ter certeza
• ter certeza de que...
• ter certeza disso

You use de when certeza is followed by what you are certain about.

Examples:

• Tenho certeza de que ele vem.
• Tenho certeza do resultado.
• Tenho certeza disso.

But if the sentence stops at certeza, there is no need for de:

• Tenho certeza.

So de is not part of the short sentence itself; it appears when more information comes after it.

Yes. That is also correct.

• Estou certo = said by a man
• Estou certa = said by a woman

This means something like I’m sure or I’m right/certain depending on context.

Compared with Tenho certeza:

• Tenho certeza is extremely common and works for everyone
• Estou certo/certa agrees with the speaker’s gender
• Estou certo/certa can sometimes sound a bit more like I’m convinced or I’m right

So Tenho certeza is often the easiest and most neutral choice.

A rough pronunciation guide is:

• tenho ≈ TEN-yoo
• certeza ≈ sehr-TEH-zuh or sehr-TAY-zuh, depending on accent

A few key points:

• nh in tenho is like the ny sound in canyon
• the c in certeza sounds like s because it comes before e
• the r in Brazilian Portuguese varies by region, but in many accents it sounds somewhat like an English h or a soft guttural sound

A rough full approximation:

• TEN-yoo sehr-TEH-zuh

It will not be exact, but it helps as a starting point.

It is neutral and very common in both spoken and written Brazilian Portuguese.

You can use it:

• in casual conversation
• in professional situations
• in writing
• as a short answer

For example, it works well as a reply to a question:

• Você vai mesmo?
• Tenho certeza.

So it is a very useful everyday phrase.

Very simply:

Negative

• Não tenho certeza. = I’m not sure.

Question

• Tem certeza? = Are you sure?
• Você tem certeza? = Are you sure?

In everyday Brazilian Portuguese, dropping você is very common when the meaning is clear, just like dropping eu in Tenho certeza.

So these patterns are worth learning together:

• Tenho certeza.
• Não tenho certeza.
• Tem certeza?