"Estar" is used for:
* Location: "Estoy en casa" (I am at home).
* Physical and emotional states: "Estoy cansado" (I am tired), "Estoy feliz" (I am happy).
* Temporary conditions: "El agua está caliente" (The water is hot), "Estoy enfermo" (I am sick).
* Progressive tenses: "Estoy hablando" (I am talking).
"Ser" is used for:
* Permanent characteristics: "Soy alto" (I am tall), "Ella es doctora" (She is a doctor).
* Identity: "Soy americano" (I am American).
* Time and date: "Es mediodía" (It is noon), "Es lunes" (It is Monday).
It's important to remember that "ser" and "estar" are not always interchangeable. To understand the proper usage, you need to consider the context and the nature of the "being" you're describing.