In how many ways can 21 books on English and 19 books on Hindi be placed in a row on a shelf so that two books on Hindi may not be together?

There are 40! arrangements of all 40 books. To find the arrangements where two Hindi books are together, consider them as one unit. There are 20! ways to arrange this combined unit along with the other books. Then, within this unit, there are 2! ways to arrange the two Hindi books. So, the total number of ways where two Hindi books are together is 20! * 2!. Subtract this from 40! to find the total number of ways where two Hindi books are not together.