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it is since和it is before的区别 it is before 1）It will be/＋时间段＋before… It will be two years before he leaves the country. 2）It was +时间段+before… It was three weeks before he came back. 3）It will not be + 时间段 + before… It won’t be long before she comes back. 4）It was not +时间段 +before… It wasn’t long before he left the country. 从句和主句的时态： ① 一致：过去式 ② 时态退一格：主句为将来时，从句为一般现在时 it is since It is/has been＋时间段＋since…如： It is/has been three years since she joined the army. 主句和从句的时态： 1) 从句常用一般过去时，主句常用现在完成时或完成进行时（表示时间段时，可用一般现在时）。 2) 固定句型：It is/has been＋一段时间＋since＋一般过去时。 |
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