yes

[jes]
particle, noun oui; (in reply to negative question) si
to say yes — dire oui
the yeses and the nos — les oui et les non
••
yes is translated by oui, except when used in reply to a negative question when the translation is si or, more emphatically, si, si or mais si: ‘did you see him?’ - ‘yes (I did)’ = ‘est-ce que tu l'as vu?’ - ‘oui (je l'ai vu)’; ‘you're not hungry, are you?’ - ‘yes I am’ = ‘tu n'as pas faim?’ - ‘si (j'ai faim)’
Note that there are no direct equivalents in French for tag questions and short replies such as yes I did, yes I am
For some suggestions on how to translate these, see the notes at do and be