ser vs estar
Both mean 'to be' — but ser is identity, estar is state. The rule about permanent vs temporary will get you in trouble.
ser
Ser encodes what something fundamentally is — identity, category, origin, material.
Es aburrido.
He's boring (as a person).
His personality. Swap to está and he's just bored right now — totally different verdict.
La boda es en la iglesia.
The wedding is at the church.
Location of an event uses ser, not estar. Catches everyone out.
Es muy guapa.
She's beautiful.
Appearance as identity. But 'está muy guapa hoy' means she looks great today — she made an effort.
estar
Estar encodes current state — how something is right now, not what it is.
Está muerto.
He's dead.
Death is permanent but it's still a state the body is in. 'Es muerto' doesn't exist.
Este café está muy bueno.
This coffee is really good.
Quality you're experiencing right now. 'Es bueno' is a general verdict; 'está bueno' is tasting it.
Está listo.
He's ready.
Versus 'es listo' = he's clever. Same adjective, completely different meaning depending on verb.
Tricky bit
Adjectives that flip meaning: listo (clever/ready), malo (bad/sick), bueno (good/tasty), vivo (sharp/alive), seguro (safe/certain).