When do you say “si” instead of “oui” in French?

French has three answer words, not two. Oui agrees with a positive question; si contradicts a negative one. Tu ne viens pas ? — Si ! means “Yes I am.” Use oui there and you have said something confusingly close to nothing. It also overrides a refusal: somebody turns you down, you come back with si, and you have insisted without needing another word.

Here a man declines a drink — Non, c’est gentil — and gets si straight back, followed by ça vous fera du bien, ça vous changera les idées (“it’ll do you good, it’ll take your mind off things”; changer les idées is the standard French for taking someone’s mind off something). German pulls the same trick with doch and Dutch with jawel; English has to spell it out — “yes I am,” “yes it does.”