Hi Paulenrique - to me, when using achar for 'think' (instead of 'find') it is usually for an opinion instead of a thought (pensar). Example: "Acho que sim.". In English that means "I think so." or more literally "I find that true.", which would be my opinion. I do not know if that is technically correct, but it has helped me differentiate the two. I guess my personal rule on this is that if I can substitute the word "find" for "think" and the sentence still makes sense, then I use "achar".