The same as "ser" and "estar" (to be), it can be hard to understand the difference if your native language is English. The verb "pensar" can be used as the verb to think or to thought. Ex: Let me think about it. - Deixe-me pensar sobre isso. He thought of his mother. - Ele pensou em sua mãe. I'm thinking. - Estou pensando. Think more. - Pense mais. And "achar" can also be used as "to think", but in a "guess way". It can also mean "to find". Ex: I think it'll be nice. - Acho que vai ser legal. It can be done, I guess. - Isso pode ser feito, eu acho. He found a wallet in the street. - Ele achou uma carteira na rua. What do you think. - O que você acha.