* Acceleration is the rate of change of velocity. This means acceleration tells you how quickly the velocity is changing.

* Velocity is a measure of both speed and direction. So, a change in velocity can mean a change in speed, a change in direction, or both.

* Zero acceleration means the velocity is not changing. If the velocity is not changing, then the speed (the magnitude of velocity) is also not changing.

Therefore, if a particle has zero acceleration, it cannot speed up. It might be moving at a constant speed, or it might be at rest, but it won't be increasing its speed.