Big Idea
"Ser" vs. "Estar" is about choosing the word that matches the thought.
Verbs are the engine of a sentence. Start with the moment in time, then choose the form that carries that moment.
Examples
Example 1
Ella es médica.
She is a doctor.
Notice es: Identity or defining role.
Example 2
Ella está cansada.
She is tired.
Notice está: State or condition.
Example 3
La fiesta es en casa.
The party is at home.
Notice es: Event location uses ser.
Step 1:Do not ask which English word matches.Step 2:Ask what the speaker means: identity, state, destination, cause, request, or result.Step 3:Choose the Spanish word that carries that meaning.
Watch for this: The trap is translating word for word. Spanish choices usually come from the relationship between ideas, not from a single English gloss.
With "Ser" vs. "Estar", begin with meaning and time. The ending is there to serve the sentence.