One of the nemeses of beginning (and even not-so-beginning) Spanish students is the existence of two words, ser and estar, for the ubiquitous verb "to be." A simple and frequently given explanation is that estar is used for temporary states (mood, condition, etc.) and ser for permanent features (physical description, nationality, etc.). But as with all simple rules, this one is loaded with exceptions. Ser is also used to describe a person's profession---hardly permanent in this day and age---but estar to indicate whether he is dead or alive!
A more accurate explanation is that ser refers to the intrinsic characteristics of a person or object, whereas estar describes his condition. Say to a woman "Eres bonita" (from ser) and you're telling her she's beautiful, but say "Estás bonita" (from estar) and you're paying her the somewhat backhanded compliment "You're beautiful [today]," implying that she usually isn't.
I was thinking about all this again this evening when my delightful, nutty daughter was hamming it up in her bath. I usually speak Spanish to her during this time, so I found myself laughing, "Kiera, estás loca....no, ¡eres loca!" She wasn't just being crazy, she is crazy.
Usually, the features of a foreign language that give us the most difficulty are precisely those same features that allow us to express in them certain sentiments with so much more elegance than we can in English. I'll share more examples in the future.