While both ser and estar can translate to to be in English, they aren't interchangeable in Spanish.
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.
Temporary Physical States
Temporary Mental States
A helpful tip for remembering conditions is to ask yourself Can I feel ____ ?
- If the answer is yes, you should probably use estar.
- If the answer is no or not really, you should probably use ser.
For example, 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. If you're not sure if you should use estar or ser with the adjective rubia(blonde), ask yourself Can I feel blonde? and answer Not really. So you use ser: