There are several ways to explain the specific uses of the verb Ser vs Estar. However, I'll give you a very good advise about it. We basically never use the verb Estar followed by a NOUN. It could be an adjective (cansado, triste) another verb (hablando, comiendo) or a preposition followed by a place (en la cocina, en la oficina), by the other hand, only the verb Ser is used to IDENTIFY two NOUNS, for example, "Yo soy doctor", "Ella es mujer", "El gato es un animal". So, observe how the pronouns yo, ella are also NOUNS and in the three examples I'm giving Ser is been used it to IDENTIFY them...