How do you know when to use ser or estar when saying to be?

Ser is used when what is described is considered a permanent characteristic and estar is used when what is described is considered a temporary characteristic. Estar can be used when what is being described can change easily. For example, if you say "Estoy aburrido" you imply that you are bored right now. However, if you say "Soy aburrido" you imply that you are a boring person in general (you being boring is just part of your personality).

Ser is also used to tell time. Some may mistakingly feel that because the time constantly changes that estar should be used. However, when you say "It's one o'clock," that is a fact and can not be changed.

Many people like to break up when to use ser and estar into the following categories. However, the main concept is that ser is used to imply permanence and estar for temporary circumstances.

Ser - time, personal characteristics, nationality

Estar - feelings, location