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［误］ While I was walking along the street yesterday, I met an old friend.
［正］ When I was walking along the street yesterday, I met an old friend.
［析］ 这里用when表达在一个动作的进行中，另一个动作突然发生了。正在进行的动作用一进行时态，而突然发生的动作用一般时态。
［误］ While I heard the bad news I felt sad.
［正］ When I heard the bad news, I felt sad.
［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，而这个时间点是主句动作的启始点，所以主句一般要用完成时态。
［误］ Because he didn't study hard, so he didn't pass the exam.
［正］ He didn't pass the exam because he didn't study hard.
［析］ because 与 so在英文中两者不能并用的，只能在句中用其一。
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