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对很多人来说，时态是他们学习英语道路上的一个拦路虎，大家都知道使用汉语句子表达时不需要考虑动词时态，但英语和汉语完全不同，不仅简单句要考虑时态问题，复合句更要考虑时态问题。
例如时间状语从句和条件状语从句，这两种复合句的时态就要遵循主将从现的语法规则，而主将从现规则常见有三种句型结构，具体内容如下：
句型结构一：主句是一般将来时态，从句用一般现在时态
例句：Mary won't ask you to go to the park with her if you are busy doing homework.
译文：如果你忙于做家庭作业，玛丽就不会让你和她一起去公园。
例句：Mike is going to be a scientist when he grows up.
译文：迈克长大时打算成为一名科学家。
例句：I will call you back as soon as he leaves for Beijing.
译文：他一动身去北京，我就会给你回电话。
句型结构二：主句是祈使句，从句用一般现在时态
例句：Don't forget to eat breakfast before you go to school in the morning.
译文：早上上学之前不要忘记吃早餐。
例句：Never bother her when she practices playing the piano.
译文：当她练习弹钢琴时，绝不要打扰她。
例句：Please look them up in the dictionary if you don't know these new words.
译文：如果你不认识这些新单词，那就请查阅一下词典吧。
句型结构三：主句谓语有情态动词，从句用一般现在时态
例句：Mary can't go on a picnic with them if she has a lot of work to do this weekend.
译文：如果玛丽这个周末有许多工作要做的话，她就不可能和他们一起去野餐。
例句：You mustn't leave the classroom in advance unless your teacher gives you his permission.
译文：除非你获得老师的同意，否则你不得提前离开教室。
例句：They may go on a trip to the Great Wall for free as long as they finish the work on time.
译文：只要他们按时完成工作，他们就可以免费去长城旅游。
以上就是有关主将从现的三种句型结构，应用的范围就是状语从句，大家一定要搞清楚，千万不要用在其它类型的复合句身上。
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