when 表示“何时”“什么时候”时，可用于过去时态或将来时态，但通常不用于现在完成时。如：
你什么时候和他首次见面的?
误：When have you first met him?
正：When did you first meet him?
但是，since when 可以与现在完成时连用。如：
Since when have you lived here? 你从什么时候起住在这里的?
Since when have you become active? 你什么时候开始变得积极了?
Since when do you read newspapers? 你什么时候开始读报的?
That was in 1949, since when things have been better. 那时是1949年，从那以后情况就好些了。
