when,while和as的用法区别
①三者均可表示“当……的时候”,如果主句表示的是短暂的动作,而从句表示的是一段时间,三者可通用.如：
I met Kang Li as／when／while I was walking along the street．当我沿街散步时碰见了康丽.
②when可以和延续性动词连用,也可以和短暂性动词连用；而while和as只能和延续性动词连用.如：
It was snowing when we got to the airport．当我们到达机场时,天正下着雪.（不能用while）
③as强调主句与从句表示的动作同时发生,as常表示“随着……”；“一边……,一边……；while强调主句表示的动作持续于while所指的整个时间内；when可指主、从句所述动作同时或先后发生.如：
As the time went on,the weather got worse．（as表示“随着……”之意）
He sang as he went along．他边走边唱.
Please write while I read．我读的时候,请写下来.
When he reached home,he had a little rest．回到家后,他休息了一会儿.
④when用于表示“一……就……”的句型中（指过去的事情）.
somebody had hardly（＝scarcely） done …when．．．＝Hardly ／ Scarcely had somebody done …when．．．
①I had hardly ／scarcely closed my eyes when someone knocked at the door．＝Hardly ／ Scarcely had I closed my eyes when someone knocked at the door．我刚一闭上眼,就有人在敲门了.