"I am thinking of that beautiful face."
Translation:Myslím na tu krásnou tvář.
I wrote, "Myslím si o tamtom krásném obličeji," and was told that that's wrong, and the correct answer is, "Přemýšlím o tamtom krásném obličeji."!
Your answer was rejected and rightly so.
The reflexive verb "Myslet si" is "think" but only in the meaning "having/expressing an opinion". See examples below:
Myslím si, že máš pravdu = I think you are right.
Myslím si o něm, že je hloupý = I think that he is stupid (My opinion of him is that he is stupid)
On the other hand "think of/about" can be translated as "myslet na" or "přemýšlet o".
I must be missing something obvious, but why "tu krásnou" when tvář is masculine?