While both ser and estar can translate to to be in English, they aren't interchangeable in Spanish.
Ser is used to classify and identify permanent or lasting attributes.
Estar is used to indicate temporary states and locations.
A simple way to remember some of the uses of estar is to think of the mnemonic device (something that helps you remember information) LoCo, which stands for Locations and Conditions. This article covers conditions. Let's take a look!
Using Estar with Conditions
Estar is used to express feelings or temporary physical condition. A condition can be an emotion, a temporary physical state, or a temporary mental state, among others.
Emotions
Estoy triste.
I am sad.
Ella está contenta porque recibió unas flores de su novio.
She is happy because she received some flowers from her boyfriend.
Temporary Physical States
Estoy cansada esta mañana.
I am tired this morning.
Mis niños están enfermos hoy.
My children are sick today.
Temporary Mental States
Mi padre está un poco loco.
My father is (acting) a little crazy.
Los estudiantes están muy confundidos.
The students are very confused.
Tip Time
A helpful tip for remembering conditions is to ask yourself Can I feel _ ?
If the answer is yes, use should probably use estar.
If the answer is no or not really, you should probably use ser.
Example:Triste
You're not sure if you should use estar or ser with the adjective triste (sad). You ask yourself Can I feel sad? and answer Yes! So you use estar: Estoy triste. (I'm sad.) Morena (Brunette)
Example: Rubia
You're not sure if you should use estar or ser with the adjective rubia (blonde). You ask yourself Can I feel blonde? and answer Not really. So you use ser: Soy rubia. (I'm blonde.)