现在完成时语法复习
现在完成时是一种英语语法结构，用于描述过去发生的动作对现在造成的影响或结果。该语法结构的构成是助动词 have/has 加上过去分词。
肯定句的结构是“have/has + 过去分词”，其中 have 用于第一人称和复数人称，has 用于第三人称单数。例如：
* I have eaten supper.
* He has eaten supper.
* They have eaten supper.
否定句和疑问句的结构与肯定句相似，只是将 have/has 改为 hasn't/haven't 和 has/he has 等。例如：
* I haven't eaten supper.
* Has he eaten supper?
* He hasn't eaten supper.
现在完成时态的分类有两种用法：
用法一：表过去发生的某一动作对现在造成的影响或结果
* It's so dark. Someone has turned off the light. (有人刚把灯关了，现在很黑)
* I have just had my breakfast. (对现在的影响：不饿)
* Has he had his breakfast? Yes, he has.
通常与表示包括现在在内的时间副词连用，如 just, already, before, yet, never, ever 等。例如：
* I have never heard of that before.
* Have you ever ridden a horse?
* She has already finished the work.
用法二：表示过去已经开始，持续到现在的动作或状态
* 该结构用于描述过去已经开始，持续到现在的动作或状态，也可以表示一直持续到现在并可能还将持续下去。
* 动词通常使用延续性动词，如 live, work, study, learn 等。
* 该结构常与表时间段的时间状语连用，如 for, since, so far, up to/till now, in the last/past+ 一段时间等。例如：
* I have lived in Beijing for five years.
* He has been studying English since 2010.
* She has been working here for three months.
现在完成时态的练习题：
1. I ________ (have) lunch already.
2. Has the train _______ (arrive) yet?
3. Tom ____ never ____ (be) to China.
4. The twin ______ just _____ (see) my father.
5. The twins ____ (see) my father just now.
6. ____ you ever _____ (ride) a horse?
答案：
1. have had
2. has arrived
3. has never been
4. has just seen
5. have seen
6. have ridden
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