They are interchangeable but not in every single case. Achar expresses opinion (anytime you can replace "think" by "find", use achar). Pensar = to think, meditate, pound over.
"Pound over" I have never heard, but sometimes people say, "mull over".
Melhor atrasado do que nunca rsrsrsrs. I didn't even have a smartphone back in 2013, I was using Duolingo on my laptop kkkkkk. But I understood you, Paul. And thanks for all your help during these past five years. Thanks for all your help too Emeyr.
I read somewhere that "achar" is more like "to guess", whereas "pensar" is "to think", but I was marked wrong translating it as "I guess". Should that be reported?
Why no "it" at the end of this one? Is DL trying to drive us crazy, or is there some method to their madness. DL usually insists on this optional and sometimes totally unnecessary "it"
Remembering is an action you can do generally. So whike you can remember some thing, or someone, you can also simply remember. If you want to say "I remember that (this, it, him, her, them, etc)" then you would need isso (isto, o, -lo, -la, -lhes...). But if you are simply saying you remember, without any indication of a specific thing you are remembering, then the article can be left out.
I think he meant ponder over. Eu tava pensando muito ontem.. I was really thinking a lot yesterday...
I think in this case you just don't need "it". And there isn't one in the portuguese example. Wouldn't it be "o" o " lembra-o" ?