一、基本用法
when 表示何时什么时候时，可用于现在时、过去时态或将来时态等。如：
When does the train leave? 火车什么时候开?
When did you see her last? 你上次是什么时候见到她的?
When did you arrive in Europe? 你什么时候抵达欧洲的?
When do you think it can be operated on? 你想什么时候可以给它动手术?
When is the contract effective from? 合同什么时候开始生效?
When does she visit you here again? 她什么时候再来这里看望你?
When are you due to visit the dentist next? 你下次什么时候该来看牙医。
When are you going to get your haircut? 你准备什么时候去理发?
但when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?
二、特别注意
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?
Since when have you been working here? 从什么时候开始你在这儿工作?