Translation:Žofie thinks that spiders are beautiful.
The Leda Česko-Anglicky Slovník gives "pretty" as one translation of "krásný"
Its because pavouci is masculine animated (ma) and here in plural nominative. -y adjectives will then always end with -i. Krásní, hezkí, dobrí,...
The point is that pretty is not considered a synonym of beautiful and is not accepted here.
I would like to know this, too. Seems like "si" is there sometimes, and then sometimes it's not. If the explanation is in a Tips and Notes, could someone point me to the right one, please?
The difference between "myslet" and "myslet si" is quite subtle and you can safely use both here. But consider "myslet" and "myslet si" as two very closely related verbs, one non-reflexive and one reflexive.