creo que
means 'I think'
* Creo que sí
I think so
* Creo que ha sido Didiaj
I think it was Didiaj
*Pienso que (means creer ,opinar)
to think about something as in giving one's opinion
* Pienso que no vendrá
I don't think she'll come
Notice ,that " Creo que" is definitely more popular than "pienso que" when you want to say "I think, believe, have the opinion of ...."