Question

Point O is between points Mand N on line segment MN. O is the midpoint of line segment MN if

ON = NM
OM = NM
ON + OM = MN
ON = OM

Answer 1

Diagram:
M------------O-------------N
................. ^ midpoint
Obviously if O is the *mid*point, then it is exactly halfway between M and N. That also means it has bisected the segment into two equal parts.
On one side you have MO (or OM, if you like)
On the other side you have ON.
In order for O to be the midpoint:
ON = OM