Let’s start by thinking of directed numbers as a lift in a huge building. The lift goes both above the ground, that’s positive, and below the ground which is negative. Ground floor is called zero, it’s neither positive, nor negative. It’s our starting point.
If a number is positive, the lift goes up. Do you remember that we assume that a number is positive unless it has a minus sign in front of it. If the number is negative, the lift goes down.
So if we look at this expression five minus eight. We assume that the five is positive, positive five means that we’re going up in the lift, we go five floors above zero. Minus eight means we’re going down eight floors, so down we go below the ground or below zero. So the answer will be negative, we end up three floors below the ground on minus three.