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.

Pensar = think Eu PENSO sobre isso o tempo todo = I THINK about it all the time Achar = guess Eu acho que ele fez alguma coisa. = I guess he did something. "Achar" can be used as "Find" too. Eu vou achar o dinheiro. = I will FIND the money Achar (doubt, when you are not sure about something) = guess Achar/Encontrar = To find. I hope this could help you :)

Additional information: The verb ACHAR can also mean "to SUPPOSE" (supor - em português). I suppose so - Eu acho que sim. Do you think it's going to rain?—I suppose so / Você ACHA que vai chover? - Eu ACHO que sim.

First, it's interesting to see the possible translations for each one according to the dictionary: - PENSAR: 1) to think 2) to have an opinion 3) to ponder, to meditadte, to reflect 4) to imagine, to suspect, to suppose 5) to consider, to intend - ACHAR: 1) to find 2) to suppose, to believe, to guess You see that the main meanings are totally different ("pensar" = "to think" and "achar = to find). But in some situations both has the same meaning that express the idea of "to suppose", "to believe", "to have an opinion". Examples with PENSAR: - "O que você está pensando?" (What are you thinking?) - "Pensei muito sobre este assunto." (I thought a lot about this subject.) - "Eu pensei que você fosse minha amiga." (I thought you were my friend.) - "Eu nunca pensei em fazer isso." (I never thought about doing that.) - "Pensei que devíamos ir." (I thought we should go.) Examples with ACHAR: - "Achei o que queria." (I found what I was looking for.) - "Dificilmente você achará coisa melhor." (You will hardly find something better.) - "Acho difícil acreditar." (I find it hard to believe.) - "Eu acho que não." (I do not think so.) - "Que acha do livro?" (What do you think of the book?) - "Você acha?" (Do you think so?)