when、while与as这些连词均可表示“当……时”引出时间从句。
when : 所引导的从句，其谓语动词既可是持续动词，也可是短暂动词；谓语动词动作发生的时间可以与主句谓语动词的动作是同时，或之前、之后。
- She was only 20 when she had her first baby.
她生第一个孩子时只有20岁。 - He was shocked when I told him.
我告诉他时他非常震惊。
while : 句中的谓语动词应为持续动词，不用短暂动词，谓语动作发生的时间通常与主句谓语动词动作发生时间是同时。
- I read it while you were drying your hair.
在你吹干头发的时候我读了它。 - While I was in Italy I went to see Alessandro.
在意大利的时候，我去看望了亚历山德罗。
as : 侧重指从句与主句的动作同时进行，谓语动词可以是持续性的，可以是短暂性动词。
- I saw him as I was coming into the building.
我进楼时看到了他。 - He gets more attractive as he gets older.
随着年龄的增长，他变得愈发有魅力。
