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.

The indefinite article (a/an) is used before a noun in the following cases:
a) To convey the numerical sense of one

I have a pen.
There is a cat on the roof.
A hundred kilos make a quintal.

Kinds of Sentences

Simple Sentence
A simple sentence consists of a single main clause.

She wrote a letter.
They visited us yesterday.
It is snowing.
We have been waiting for hours.
Good books build character.
Birds of a feather flock together.

A sentence is the largest grammatical unit in any language. In English, a sentence always contains a subject and a predicate.