1. Sum of forces is zero <=> velocity remains constant.[If the particle is initially at rest, it will remain at rest for as long as the sum of the forces is zero].
2. Sum F of forces is proportional to the rate of change of momentum.
F = k x d(mV)/dt
F = d(mV)/dt (in suitable units)
If the mass m is constant,
F = mdV/dt = ma
3.Forces always occur in equal and opposite pairs. (To every action, there is an equal-and-opposite reaction.)