Hello, Michelle. I don't think there are rules about this, I am not sure. But we usually use "think" when we have made a mental process and "believe" when we want to express an opinion, an impression, or believes (religions, for example).

Hi again! You're right, 'Creer' is to 'Belive' and 'Pensar' is to 'Think'. For giving opinions both could be correct, and also to make predictions. But we normaly use 'Creo' for that. i Also, when you are talking about specfici belifs (religious for example), then only 'creo' is correct. And if you are talking about the action of thinking about something, then only 'pienso' is correct. If you belive in Jesus, you say 'Creo en Jesus'. If you are thinking about what to do for lunch, then you say 'Estoy pensando que hacer para almozar'. Greetings!

If you want to give your opinion about something, You can use creer and pensar They're both correct. for example. I think so: Yo creo que sí or Yo pienso que sí. In this case you should use creer if you're sure about something but wheter you aren't sure, It's much better to use pensar. next case: pay more attencion here, Yo le creo (a él o ella) = I belive him/ her. (I trust him/ her) Yo pienso en ( él o ella) = I'm thinking about him/ her. I hope this help you a little bit.