［析］ while不能表达一点儿的时间，即瞬时某一时间点。

［误］ After school some students play football, or others go to the library.

［正］ After school some students play football, while others go to the library.

［析］ while在此处意为"而，然而"。

［误］ She sang when she walked along the dark street.

［正］ She sang as she walked along the dark street.

［析］ as用于句中时，其要点是强调两个动作的同时进行。这里用when虽然不能讲是语法上的错误，但则看不出来小女孩因独自走黑暗的街道因害怕而唱歌的心情。

［误］ I finished my homework until twelve o'clock last night.

［正］ I didn't finished my homework until twelve o'clock last night. ［正］ I did my homework until twelve o'clock last night.

［析］ until用在句中时其含义是某一动作一直持续到某时结束，那么句中的动词则一定要用持续性动词，如果要用瞬间，或截止性动词时一定要用否定句式。因截止性动作的否定式应看作是持续性的动作。如离开leave是瞬间动作，因一出门即为离开了，而不离开则是长时间的。

［误］ I have studied English when I was twelve.

［正］ I have studied English since I was twelve.

［析］ since引出的时间状语从句是表达了一个时间点，而这个时间