Inertia: the force it takes to move an object at (to make it go slower, faster or in a different direction), the more mass the object has the more inertia it has (inertia happens in all cases of motion, at rest, while moving and changing direction).
Resultant force: two unbalanced forces, but when put together they are the equivalent of a single force.
Resultant force = mass x acceleration (f = ma)