"Je krásná, ale chudá."

Translation: She is beautiful but poor.

[learninggerman]

why is "she is pretty but poor" wrong?

[VladaFu]

Krásná (beautiful) is stronger than hezká or pěkná (pretty).

[AgnusOinas]

"pretty" is less beautiful than "beautiful". It would be "Je hezká" or "Je pěkná".

[AsherStern7]

I would suggest accepting also "She is beautiful, yet poor."

[AgnusOinas]

Technically that's more "Je krásná, přesto chudá", but sure, I can add it, it works.