Quick answer
ser = what something is.
estar = how something is right now, or where it is.
Forget “permanent vs temporary” — that rule breaks the first time someone dies (está muerto, which is about as permanent as it gets).
Side by side
Es aburrido.He is boring — that is what he is like.
Está aburrido.He is bored — right now, in this meeting.
Ana es guapa.Ana is good-looking — generally.
Ana está guapa.Ana looks great — tonight.
La sopa es rica.The soup is delicious — as a dish.
La sopa está rica.The soup tastes great — this bowl.
Memory trick
Ask yourself: am I describing it, or reporting on it? Describing → ser. Reporting the current state → estar. That rule survives “está muerto”, the permanent/temporary one does not.
Typical mistakes
Soy en el trabajo.
Estoy en el trabajo.
Location is always estar — no exceptions, no judgement calls.
Estoy profesor.
Soy profesor.
A job is part of who you are, even if you change it next year.
Soy cansado.
Estoy cansado.
Soy cansado means “I am tiring” — exhausting to be around.
Check yourself
¿Dónde ___ el baño?