when, while, as 都表示“当„„时候,,when 引导的从句的动作与主句的动词
同时发生,也可先后发生; as, while 引导的从句则强调主句和从句的动作同时发生
When I got to the station, the train had alreadyleft.He sang merrily as he was working.
till, until 均表示“到„„时候止”,肯定句中的谓语必须是延续性动作。
若主句谓语是终止性动词,则主句要用否定形式,意为动作“到„„才„„”开始发生
I worked till late at night.She didn't get up until her mother came in.
till 和 until 通常情况下可以互换,只是在句首时 until 比 till 更常用