English gets by with a single verb, "to be," for everything from professions to moods. Spanish splits that job between two verbs -- ser and estar -- and picking the wrong one doesn't just sound off, it can flip the meaning of the sentence entirely.
Ser is for identity -- who or what something fundamentally is. Estar is for state -- how something is right now, or where it's located.
Identity, origin, and permanent traits
Location, condition, and temporary states
Ella es aburrida, pero hoy está aburrida en la reunión.
She's boring, but today she's bored at the meeting.
Same adjective, opposite meanings -- ser aburrido describes a personality trait (boring), estar aburrido describes a temporary feeling (bored). The same trick works on listo (smart / ready) and malo (bad / sick).
Take the quiz and see if picking ser or estar has become automatic.