Smart
Used for 'clever' or 'sharp'. Be careful: when used with the verb 'estar' (estar listo), it means 'to be ready'.
Means 'brilliant' or 'very smart'. Use it to give extra emphasis, like when someone has a great idea.
My sister is very smart.
The boy is clever, he learns fast.
What a brilliant idea!
You all are very smart students.
The most important distinction is with the word 'listo/a'. 'Ser listo' means 'to be clever/smart', which is a personality trait. 'Estar listo' means 'to be ready', which is a temporary state. This is a crucial difference throughout the Spanish-speaking world.
It's very common to use 'pilo' or 'pila' to describe someone as smart or sharp, especially a good student. Example: 'Mi hijo es muy pilo en la escuela.'
Besides 'inteligente' and 'listo', you might hear 'abusado' or 'abusada' to mean 'sharp' or 'on the ball,' often in a street-smart sense.
The vocabulary is the same, but the grammar for groups is different. Instead of 'Ustedes son inteligentes', they use 'Vosotros sois inteligentes'.
The word 'vivo/viva' is often used for someone who is 'clever,' but it can sometimes have a negative meaning, implying someone is an opportunist.
Using 'estar' instead of 'ser'. For example, saying 'Él está inteligente.' — Say 'Él es inteligente.' 'Smart' is a permanent characteristic, so you must use the verb 'ser'. The verb 'estar' is for temporary states or locations.
Forgetting to match gender with adjectives. For example, 'Mi amiga es listo.' — Say 'Mi amiga es lista.' While 'inteligente' ends in '-e' and works for any gender, adjectives ending in '-o' like 'listo' must change to '-a' for feminine nouns.
Confusing 'ser listo' (to be smart) with 'estar listo' (to be ready). — These are not interchangeable. 'Mi papá es listo' means 'My dad is smart.' 'Mi papá está listo' means 'My dad is ready.' The verb choice completely changes the meaning.