pensar is when u are thinking of something,that you are using your intelligence to have an opinion about something or about a person. -eu penso que tens razão(i think that u are right)-->after you hear the person´s opinion,you pondered about it and answered that he/she is right -estou pensando em uma solução(im thinking about a solution) achar is when you say something that you are not totally sure of. -eu acho matematica dificil(i think math is difficult)-->it´s my opinion about math,that i didnt had to use my intelligence to answer it,it´s my point of view. -eu acho que ela chegará meio dia (i think she´ll arrive at mid day)->you are not totally sure if she will arrive or not i hope that helps,cheers!

Just a quick hint for the case when you are expressing an opinion or point of view... I have noticed that I usually use the verb "achar" when I wanna say "I think". Ex: Eu acho que é melhor você ir (I think you'd better go). I don't know about Portugal, but in Brazil you would rarely hear someone saying "Eu PENSO que está certo" (I think it's right). Instead you would normally hear "Eu ACHO que está certo". I usually use the verb "pensar" when I wanna say "I thought". Ex: Eu pensei que você viria com a gente (I thought you'd come with us). Another thing... In English you could say: 1) I think THAT she is still going to cause trouble or 2) I think she is still going to cause trouble In Portuguese only the first form is acceptable. So... I think THAT she is still going to cause trouble Eu acho QUE ela ainda vai causar problemas

In a very simple way, "achar" e "pensar" mean the same! However, "achar" has a second meaning! Pensar and achar both mean "to think"! Eu penso que tens razão. Eu acho que tens razão (I think you're right/ I think you have reason) The second meaning of achar is "to find"! Já achei o casaco! (I have finally found my jacket!) You can discover another thing very interesting! Is that you can translate "to find" to "achar"... I found it really interesting! --> Eu achei-o muito interessante! (but it's opinion, not something you find somewhere!) I hope you understood my speech! If you have doubts come to see me, I will give you more examples if you want :D

It's simple as the difference between think and guess... but sometimes can be used in the same way... well, I think so or I guess so! Well... Your thought is about what you think, and your guesses is about what you guess... Sounds stupid simple, but its a little bit confused... Other way... let me see... you can use guess in portuguese with the same mean as think: What do you think about my idea!? O que você acha sobre minha idéia!? You can answer something like: I think you are absolutly wrong and I've been paying a lot of attention! Eu acho que você está totalmente errado e eu estou prentando bastante atenção! If you draw it for me I can learn better, I guess! Se você desenhar isso pra mim eu posso aprender melhor, eu acho... This last one, the word guess means acho! That's it, that's all for now!

"Achar" means you are not sure about what he says and "Pensar" refers to his opinion. =D