Why should we write 9 not 10 ? And why 5 is 4 now why not 3?

This is tricky to answer, but I'll do my best.

You cant subtract 3 from 0, unless you wish to go into the negatives.

So, you have to borrow from the 0s neighbor, who is in fact another 0.

So in order to help his neighbor, the middle number 0 borrows from his other neighbor, the number 5.

5 has some to spare, so he gives the middle 0 a 1, which turns himself into a 4 and that 0 into a 10.

Then the 10 gives the first 0 a 1, turning himself into a 9 and the first 0 into a 10.

It might be confusing, but thats how it works.

Have a luvely day!