①when意为"在……时;当……时",可表示"点时间"或"段时间",从句谓语可以是终止性动词,也可以是延续性动词。例如: When I got home, he was having supper. ③while只可表示"段时间",从句谓语只限于延续性动词。例如: While I was sleeping, my father came in. 注:从句谓语是持续性动词时,when、while和as可以互换?主?从句谓语都是终止性动词时,when与as可互换?从句谓语表示状态时,通常用while。
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