Although it's useful for having some basic notion about the difference between "ser" and "estar", the idea that one is for permanent situations and the other for changeable conditions is inaccurate, and sometimes even misleading.
"Ser" is used to talk about inherent attributes of the subject, and "estar" for things that aren't intrinsic to the subject.
And although some attributes like beauty could be perceived as inherent to the person or object, they often involve a subjective perception. If the speaker doesn't think a person is especially goodlooking, but considers that they did something to look better, the verb "estar" is likely to be the chosen one.
- María es hermosa. -> María has always been perceived as a beautiful woman.
- María está muy guapa. -> Maybe I think she hasn't been cute all her life, but now she looks nice.
- Roberto es delgado. -> I'm describing Roberto as slim man, maybe because I think that's just a part of him.
- Roberto está delgado. -> Probably I met Roberto when was overweight, and now I see something happened to him that made him slim down.
Added to the previous subjective perception, "estar" can also be used when a process has made things they are:
- Estas tazas están rotas. -> Of course, this state will not change for the mugs, but something made them get broken, so it's their current state I'm describing.
- Mi madre está enferma. -> This is one of those times when a condition can change, but even if I know my mother has no cure, I will always say "está", because there was a process that made her ill and that's her current situation.
- El perro está muerto. -> That's something that will never change, but something happened that made the dog die.
For more explanations about the uses of "ser" and "estar", you might want to take a look
here.
