estan vs son
How do I know when to use which?
In general, ser (son) is used to describe a more or less permanent attribute of something. "Es bella" means "She's a pretty woman (all the time)." Estar (estan) typically describes a passing characteristic. "Esta bella," "She looks pretty (today)."
This is why you "ser" from a place but "estar" in it, as well as "ser loco" meaning you are a mentally ill person, but "estar loco" meaning you're acting crazy, often said jokingly.
The general rule of permanent/temporary will get you in trouble (sooner than) later. The best explanation of the use of SER and ESTAR I've ever read (and that helped me tremendously) is here. http://www.spanishdict.com/answers/7690/ser-estar By the way, Spanishdict.com is another great place to get some additional lessons, practice and help. I started off there and use it daily for reference.
De nada Jenny. :) Studyspanish.com is another of my favs! The Studyspanish.com starting page on ser/estar is here: http://www.studyspanish.com/lessons/serest1.htm (look for the links to all of the different lessons on the left). There are also videos and lessons at the Spanishdict.com site worth listening to and doing and some great lessons at spanish.about.com. Here is their starting lesson for ser/estar: http://spanish.about.com/cs/verbs/a/servsestar.htm which also has more links to more info. on these two verbs (and a whole lot more) at the bottom of the page. Utilize as many different free websites as you can to enforce what you learn here and find the explanations that are most helpful to your style of learning! Poco a poco... ¡Chao! Chévere
I´m new so I´m sure there is a better answer...but since no one has answered you I´ll give it a shot insofar as I under it. Ser and Estar are used instead of estan and son because they are the "root words" (Ser is the root of son and Estar is the root of estan). I hope that makes sense.