Which verb - SER or ESTAR? 1

Ser — identity, origin, material, and time — Ser is used for what something fundamentally is: identity and profession (Soy médico), origin (Es de Chile), material (Es de madera), time (Son las tres), and events with scheduled times (La reunión es a las cuatro). These are intrinsic, defining characteristics.

Estar — state, location, and progressive — Estar describes changeable conditions: temporary states (Estoy cansado), location of people and things (Está en casa), results of a change (La puerta está abierta), and the progressive: Estoy comiendo. These are all situational or changeable, not defining.

Adjectives that change meaning with ser vs estar — Several adjectives have different meanings depending on the verb: ser aburrido (to be boring — personality) vs estar aburrido (to be bored — current state). ser listo (to be clever) vs estar listo (to be ready). ser malo (to be bad) vs estar malo (to be sick). These pairs are worth memorising.