"Even I used to think that."
Translation:To jsem si myslívala i já.
You can have "Myslím, že..." (optionally "Myslím si, že...") where "že" begins another clause.
You can only have "Myslím na něco/někoho", where you cannot add "si". ((Well, "myslet si na někoho" does exist, but it's old-fashioned and means "to have romantic interest in someone.))
And finally, you can only have "Myslím si + object (often to)", where "si" is mandatory.
Yeah, it's tricky, I know :)
There's also a shade of meaning:
- Ano, to jsem si myslel. - Yes, that's what I thought.
- Jo, to jsem myslel. - Yeah, that's what I had in mind, that's what I meant.
Shown in bold is what is stressed in speech. But I'm not sure if the latter is standard and it surely doesn't fit the meaning in this exercise. So, yeah, "myslet" without "si" can be used to mean "mean" (in the sense of "have in mind") - especially together with instrumental: "Co tím myslíš?" - What do you mean by that?