SER vs ESTAR: esencia vs estado
Dannis
SER or ESTAR change the meaning: esencia vs estado (essence vs state). Quick rule of thumb: ser = identity/trait; estar = condition/situation right now.
ser listo = intelligent/clever. estar listo = ready/prepared. Example: Ana es lista. Ya estamos listos para salir.
Tip for English speakers: "soy listo" — a character trait (clever); "estoy listo" — a current state (ready).

ser bueno = kind/good-natured OR of high quality. estar bueno = in good health; tasty (for food); colloquial: attractive. Example: Mi abuela es buena. El café está bueno. Ya estoy bueno.
Careful: "estar bueno" = "attractive" is colloquial; use it carefully at work and with family.
Trick: if you can add "ahora" (now), it usually goes with ESTAR: "Estoy listo ahora". If it’s a permanent trait or a general evaluation, use SER: "Mi hermano es bueno".