From a cautious nod to full agreement:

Portuguese	English
Acho que sim.	I think so.
Faz sentido.	That makes sense.
É verdade.	That's true.
Concordo.	I agree.
Com certeza!	Absolutely!
Concordo plenamente.	I completely agree.
Você tirou as palavras da minha boca!	You took the words right out of my mouth!

You agree com someone or something: Concordo com você. Concordo com a ideia.

After an expression of doubt, concordar goes into the subjunctive like any other verb: Duvido que o Marcos concorde. — I doubt Marcos will agree.