We have seen that the past perfect tense is used to talk about a completed past action. Note that when two past actions have to be mentioned, the past perfect is used for the earlier of the two, and the simple past for the later.
The tense of a verb shows not only the time of the action but also whether it is complete or not. There are three tenses in English:
Incorrect: He has stole my pen.
Correct: He has stolen my pen.
Incorrect: John has often beat me at tennis.
Correct: John has often beaten me at tennis.
Incorrect: He told her that he will come.
Correct: He told her that he would come.
Again and again: frequently, repeatedly
We shouldn’t commit the same mistake again and again.
Now and again: once in a while, at times, from time to time, occasionally
The subject must agree with the verb in number and person. Basic rules of sentence agreement are given here.