Eu concordo totalmente.

You can absolutely say just Concordo totalmente.

In Brazilian Portuguese, subject pronouns are often omitted when the verb already makes the subject clear. Since concordo is the I form, listeners understand that the subject is I.

• Eu concordo totalmente = I totally agree
• Concordo totalmente = also I totally agree

Using eu can add emphasis, contrast, or clarity.

For example:

• Eu concordo, mas ele não. = I agree, but he doesn’t.

Concordo is the first-person singular present tense form of the verb concordar, which means to agree.

So:

• concordar = to agree
• concordo = I agree

Some other forms:

• eu concordo = I agree
• você concorda = you agree
• ele/ela concorda = he/she agrees
• nós concordamos = we agree
• eles/elas concordam = they agree

Totalmente is an adverb meaning totally or completely.

It modifies concordo, so Eu concordo totalmente means I agree totally / I totally agree.

It works very much like English totally in this sentence.

Totalmente is correct and natural, but there are other common choices.

Very common alternatives include:

• Eu concordo plenamente. = I completely agree.
• Eu concordo completamente. = I completely agree.

Among these, plenamente is especially common in more neutral or polished speech.
Totalmente is also very common and sounds natural in everyday conversation.

In Portuguese, adverbs like totalmente often come after the verb, and that is very natural here.

So:

• Eu concordo totalmente = very natural

You may also hear:

• Eu totalmente concordo

But that sounds more emphatic or stylistically marked. For a learner, Eu concordo totalmente is the safest and most natural pattern.

Often, yes. The verb concordar is commonly used with com when you say who or what you agree with.

For example:

• Eu concordo com você. = I agree with you.
• Eu concordo com isso. = I agree with that.

In Eu concordo totalmente, there is no object after the verb because it is understood from context. It is like saying I totally agree without repeating with that or with you.

Yes. Just like in English, the exact meaning depends on what comes after it.

• Eu concordo. = I agree.
• Eu concordo com você. = I agree with you.
• Eu concordo com a ideia. = I agree with the idea.

So the core meaning is to agree, and then Portuguese adds more information if needed.

A simple approximate pronunciation for an English speaker is:

eh-ooh kon-KOR-doh toh-tal-MEN-chee

A few notes:

• Eu is usually pronounced something like eh-ooh in Brazilian Portuguese.
• concordo has stress on cor: con-KOR-do
• totalmente has stress on men: to-tal-MEN-te
• In many parts of Brazil, the final te in totalmente sounds like chee

Pronunciation varies by region, but this will be understood well.

Yes. Eu concordo totalmente works perfectly as a complete sentence.

It is very common as a response in conversation, especially after someone states an opinion.

For example:

• Essa é a melhor opção.
• Eu concordo totalmente.

It sounds natural, complete, and idiomatic.

It is neutral and works in both informal and fairly formal situations.

• In conversation, it sounds natural.
• In meetings, discussions, or writing, it is also acceptable.

If you want something slightly more formal or polished, Concordo plenamente is also very common.

Usually, no—not in the same way.

For a general statement of agreement, Portuguese normally uses the simple present:

• Eu concordo totalmente.

Estou concordando is grammatically possible, but it sounds less natural in most everyday cases and suggests an ongoing process rather than a straightforward statement of agreement.

So if you want to say I totally agree, use:

• Eu concordo totalmente.