There are a few ways to express ‘Thinking’ in Korean. Today,  you are going to learn the first one and grammar point is “Plain form + 고 생각하다”.  It is very useful and high frequency. Please read examples below. FYI: the plain form is basic form of verb.

I think that she is pretty.  저는 그녀가 예쁘다고 생각해요/ jeo nun gu nyeo ga ye bbu da go saeng gag hae yo (예쁘다 is to be pretty).

I think that Korean is difficult. 저는 한국어가 어렵다고 생각해요/ jeo nun han kook eo ga eo leoyp da go saeng gag hae yo (어렵다 is to be difficult).

I think that he is the best teacher. 저는 그가 최고의 선생님이라고 생각해요/ jeo nun gu ga choi go eui seon saeng nim e la go saeng gag hae yo. (선생님이다 is is a teacher; when there is 이다 verb, you have to say 이라 right before 고 생각해요).

I think that the artist is the good person. 저는 예술가가 좋은 사람이라고 생각해요/ jeo nun ye sool ga ga jo eun sa lam e la go saeng gag hae yo. (예술가 is an artist).