Es abierta = "She is open" (i.e. "She is an open sort of person" – characteristic) Está abierta = "It is open" (probably referring to a door or window – state) Es triste = "[It/he/she] is sad" (i.e. gloomy – characteristic) Está triste = "[It/he/she] is sad" (i.e. feeling down – state) ¿Cómo eres? = "What are you like?" (i.e. "describe yourself" – characteristics) ¿Cómo estás? = "How are you?" (i.e. "how are you doing?" – state) With adjectives referring to beauty and the like, ser means "to be", and estar means "to look". ¡Qué bonita es! = "Wow, she is so beautiful" (characteristic) ¡Qué bonita está! = "Wow, she is looking so beautiful / she is done up so nicely" (state) The differentiation between "nature" and "state" makes sense when talking about the states of life and death: Él está vivo (He is alive), Él está muerto (He is dead). Estar is used for both alive and dead, since they are both states, although being dead is considered a permanent state. Ser is used when stating the stage of life at which a person is. The old, the young, etc. are seen as groups that one can belong to. It is a question of identity: Él es viejo = "He is old" Él es un viejo = "He is an old man" Cuando ella era niñita = "When she was a little girl" However, age can also be presented not as a matter of identity but a state: Él está viejo = "He is looking old" / "He got old" The use of estar gives a certain special nuance to some verbs. For example, estar guapa, though it has the sense of "to be beautiful", also emphasizes the use of make-up and clothes to create a beautiful look. Ser sucio instead of the more usual estar sucio means to be the sort of person who is likely to be dirty. The adjective loco ("mad", "crazy", "insane") is always used with estar in Spain, as the implication is that the person "has gone mad" (i.e. a change of state). It is possible to give it a permanent nuance, but only by using it as a noun: Él es un loco, "He is a madman". Ser loco is used in certain regions in Latin America, however, meaning a permanent insanity as opposed to estar loco meaning have gone mad or acting