Question

he is written a book on English grammar find the error​

Answer 1

Answer:

he is writing a book in English grammar

Explanation:

I hope this answer helps you

Answer 2

Answer:

Written is the mistake.

Explanation:

Because, here it is is which means it is talking about simple present.

But the verb is written , which is used in present perfect.

So, Instead of written write :

He is writing a book on English grammar.