What at first blush looks like the Spanish word for ‘yes’, turns out to have multiple meaning in French.
And yes, sidoes man ‘yes’ in French too, but in a contradictory sense. One uses si to say yes if you do not agree with the negative sentence that occurred just before.
“You didn’t do your homework?” Si, j’ai fait. (Yes, I did.)
However, I bet that you are already familiar with the most common use of si. No? Come on! Try harder, please. S'il vous plaît (if it pleases you). There is no such thing as being too polite. The use of s'il vous plaît should fetch you handsome returns in terms of service and attention. As will si vous voulez (If you want) in response to “Do you want to shop?” Particularly, effective with women! Believe me. I am married.
And what if you are not yet married and find some special? You might want to say il / elle est simignon. (S/he is so cute.) You could also compare. Elle n'est pas simignon que d'elle. (She is not as cute as her). Commencer datation, sinon quelqu'un d'autre le fera. (Start dating, otherwise someone else will.)
Although sinonis used in the sense of otherwise in the above sentence, the root of sinonseems to lie in si and non(if not).