"He thinks that I am already sleeping."
Translation:Myslí si, že už spím.
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 - this link is very helpful - although the first few comments claim that there is "no discernible difference" between the two, another explanation seems clearer: "Myslet" is used when thinking ABOUT something, and "Myslet si" is when someone has an opinion on something. Would you agree?
There are (at least) four possibilities:
- The verb is followed by another clause "..., že..." - we can use both "myslet" and "myslet si" and I don't feel there's a difference.
- The verb is followed by "na" (to think of/about) - do not use "si", use non-reflexive "myslet".
- The verb is used as "to mean (+ by something)" - e.g. "Co tím myslíš?" (What do you mean by that?) - use non-reflexive "myslet".
- The verb is followed by an object, often "to", for example "Taky si to myslíš?" (Do you also think so/that?) - always use the reflexive "si".