It's not always... but being straightforward, "pensar" means "think about", "to reflect", and "achar" means only "to think", to give your opinion. When you say "pensar" you literally get a time to think deeper about it,... when you say "achar", it's your opinion and you don't necessary think so much about it(although you might have done it). See the examples bellow. Example 1: Thinking about this topic, I think you're right - Pensando sobre esse assunto, eu acho que estás certo. Example 2 (conversation): Person A: What do you think about Brexit? O que você acha sobre Brexit? (Person A is asking the other's opinion) Person B: I don't know, I'll think about it Eu não sei, vou pensar sobre isso (Person B doesn't have an opinion, she/he needs to deeply think, to reflect about this topic) Could I clarify a little?

Both words could mean "think", but "achar" actually means "to find". So, in a formal (especially written) conversation, you always should use "pensar" when you're talking about "thinking" and "achar" only when you're talking about "to find" something. During an informal conversation, you may use both. Although, "achar" meaning "think" should always be avoided. I hope it helps! :)