800-675, why do we borrow?

The reason why we borrow is because you cant subtract 0 from 5

When doing vertical subtraction, 800 - 675 will turn out like this:

800

- 675

You should know that we start on the right and then end on the left.

So, we do 0 - 5, 0 - 7, then 8 - 6, right?

No. We have to borrow because you can't have a negative number when doing each number individually, only until the end. In this case, there won't be any negative numbers.

You can't do 0 - 5 or 0 - 7, so you have to borrow, changing the number to the left of it one lower (unless it's 0, so then you must borrow the the next number after that).