Learn Portuguese (Portugal): Estou de acordo contigo.

In Portuguese, agreeing is viewed as a temporary state of mind or an opinion about a specific topic, so we use the temporary verb estar (to be). You are currently "in agreement" for this moment.

Is there a single verb for "to agree" instead of saying estar de acordo?

Yes! You can use the verb concordar. Saying concordo contigo means exactly the same thing as estou de acordo contigo. Both options are very common and natural in Portugal.

Where does the word contigo come from?

It is a special fusion. When the preposition com (with) is combined with the informal singular "you" (ti), it always merges into the single word contigo. You can never say com ti or com tu.

How would I change this to say "I agree with that"?

You would say estou de acordo com isso. The preposition com stays as a separate word here because it only fuses into a special single word when used with certain personal pronouns, like mim (comigo) or ti (contigo).