"He thinks that I am already sleeping."
Translation:Myslí si, že už spím.
My answer -- Myslí, že už spím -- was wrong, because I left out "si." But I don't understand when the verb should be "myslet si" and when just "myslet" is fine. If someone could explain, that would be great!
I think you can use both here. There is some discussion about the difference at https://forum.wordreference.com/threads/myslet-si-myslet.420478/
Presently we do accept On myslí and we do not accept just Myslí. It is probably just missing.
Thank you, especially for the link! Should I report it, so that it can be added, if I get this exercise again?