I did "they are pretty cities." It isn't perhaps the most common way of saying it, but it works. I think "to" indicates an actual set of things , so in English we would use "They are " as opposed to the existential existence of pretty towns, which would be "there are". I will let a good Czech speaker answer how to say "There are pretty towns" in Czech.
I said "The cities are nice" and it corrected me with "They cities are nice". "They cities are nice" does not make sense. Can someone explain this to me, please? EDIT: I assume it should be "those" cities are nice. But why is it telling me to say "they". That does not make sense. They should not be a correct translation, it should be those.
In this exercise the subject noun is "to," "those" and the object is "hezká města," "nice towns." In your version, you're making the subject "these towns/those towns," which is a different sentence structure, and the "to" must decline to "ta" to match the plural form of města. Ta města jsou hezká. That's a different sentence even though the same idea is depicted.