When do you use estar ?

Answer 1

http://www.spanishdict.com/conjugate/estar

Answer 2

Estar is used when you are talking about something that won't last forever. You would say "estoy cansado" if you're tired, not "soy cansado" because you won't be tired forever. You would say "soy Americano" not "estoy Americano" because your nationality is permanent.