一个动作可能发生于现在，过去，或将来。
英语中，动词的时态描述了动作发生的时间。
| 现在 | 过去 | 将来 | |
|---|---|---|---|
| 一般(表明动作发生的时间) | finish | finished | will finish |
| 进行 | am/is/are finishing | was/were finishing | will be finishing |
| 完成 | have/has finished | had finished | will have finished |
| 完成进行(发生在过去,持续了一段时间.可能在过去结束，也可能在现在或将来结束) | have/has been finishing | had been finishing | will have been finishing |
有些比较难理解的时态，我们单独解释一下：
现在完成进行时
- It has been raining, and the street is still wet.
- I have been running, and I am still tired.
- She has been practicing the piano, and she is much better now.
现在完成进行时，描述了一个动作，开始发生和结束都在过去，持续了一段时间；但是，这个动作的影响延续到现在。
过去完成进行时
- It had been raining, and the street was still wet.
- I had been running, and I was still tired.
- She had been practicing the piano, and she had gotten much better.
过去完成进行时描述了一个持续的动作，开始发生在过去，在另外一个过去的动作前完成。
未来完成进行时
- By tonight, it will have been raining several hours, and the street will be very wet.
- By next summer, I will have been running for almost a year, and I will be fit and healthy.
- By the time of the concert, she will have been practicing the piano for several months, and she will be much better.
未来完成进行时，描述了一个持续的动作，将要完成在未来。
练习
https://www.jianshu.com/p/74d5462fe123
参考
http://www.talkenglish.com/grammar/simple-tense.aspx
http://www.talkenglish.com/grammar/perfect-progressive-tense.aspx
