Which preposition to use with quand

on Occurrences 57%

Of this able woman, it was said by Talleyrand: 'Elle est toute assertion; mais quand on demande la preuve c'est son sécret' Of Lord Holland, the keen diplomatist observed: 'Cest la bienveillance même, mais la bienveillance la plus perturbatrice, qu'on ait jamais vue.'

Which preposition to use with  quand