When you use "ter certeza" you are 100% sure of something, there is no doubt of that. For example: "Eu tenho certeza de que vou ser demitido": I am sure I am going to be fired. Maybe you heard it from the HR or something else, but you know it for a fact. When you say "achar" you are not 100% sure - it can range from 1 to 99%. You use the verb "achar" when you want to share your opinion about something. For example: "Eu acho que vai chover hoje" (I think it will rain today): you think it might rain, but isn't sure. "Eu acho você legal" (I think you are cool): it's my opinion of you. When you use "pensar", you mean "to reflet on something", "to ponder". It means you had some time to think about someone or something. Example: "Nós pensamos que você ia gostar deste prato" (We thought you would like this dish): We had thoughts and podered that this dish would be good.