The usual form of the affirmative, yes, in French, is "oui."

But my understanding is that there is another form of the affirmative, "si," (similar to the Spanish). It is used only in limited contexts, and is perhaps a different, "weaker" way of saying "yes."

In Japanese, for example, there is a word "hai," which is a "conditional" yes.

In what contexts are "si" used, and how does it differ in nuance from "oui"?

The usual form of yes in French is "oui."

But my understanding is that there is another form of the affirmative, "si," (similar to the Spanish). It is used only in limited contexts, and is perhaps a different, "weaker" way of saying "yes."

In Japanese, for example, there is a word "hai," which is a "conditional" yes.

In what contexts are "si" used, and how does it differ from "oui"?