Newton's second law of motion is the less force applied, the less the object moves. The equation for this law is f=ma. The "f" is force, "m" being mass, and "a" being acceleration. So the equation would really be force equals mass times acceleration. If you use more force on an object, the acceleration increases. If the mass increases but the force stays the same, then the acceleration will decrease.

Inertia is the natural tendency for an object to resist a change in motion. Inertia and mass are related because the more mass an object has, the more inertia it will have. An example would be a human not being able to move an elephant by himself/herself. This relates to Newton's three laws of motion because it is almost the entirety of Newton's First Law.