Sun_kim - forget about that "temporary/permanent" business with ser/estar ; that will just confuse you more. Professions/occupations always get the SER, and so are general descriptions, points of origin, characteristics or essence, time expressions & dates/numbers (mathematical expressions), where an event takes place, and identification [It's a table].
Use ESTAR for (temporary) state/conditions & emotion [ripe, overcooked, sick, happy, etc], personal opinion [He's ugly in those pants.], action (present progressive/happening-now verbs) [sitting, standing, etc], result of a process [broken, dead, etc], and where things are physically or geographically located.
Yes, something like that :). 'Mi hermano es feo' is definitely not wrong; it's simply stating a fact that my brother is indeed an ugly man; defining him, as in he's tall, dark, but not handsome. 'Mi hermano está feo' would be saying something like 'my brother looks (is looking) ugly' [today]'.
Another response I read that helped me remember the distinction: YOU are more important than what is YOURS. Therefore YOU have the emphasis (accent on YOU/TÚ) and not YOURS. Of course this is only a trick to help remember and is not absolutely literal. I hope it help someone else, too.
It is accentuated when it indicates the person we are talking to: tú vienes (you come); entre tú y yo (between you and me). It is not accentuated when we mean that something is owned by the person with whom we speak: tu carro (your car); tu forma de hablar (the way you talk). The plural of tú is ustedes, while the plural of tu is tus: ustedes vienen (you come); tus carros (your cars).
The diacritical tilde also has the same effect on other monosyllables: Él anotó el gol (he scored the goal); el té rojo es el que más te gusta (Red tea is the one you like the most).
Hola Amiga Periana: No. "Eres" is present tense, second person, singular. It has to be "eres" because "eres" goes with "tú". If you use "es", it has to go with "Usted". They are both present tense. Use "tú" when talking informally, to a child, or to a subordinate. Use "Usted" when talking formally, to someone you just met, to a superior, etc.