If the acceleration of a system is zero, are no external forces acting on it? what about internal forces? explain your answers.

Yes

May not be zero

Explanation:

a) When a acceleration of a system = 0

F = m*a = 0

Hence,

All resultant external forces are zero

b) A system may consists of multiple masses or objects interacting with one another. Hence, those internal are non zero for the system to remain intact

E.g two boxes riding on one another moving at a constant speed.

Hence, a single object system has zero internal forces; however, a multiple objects system will have non zero internal forces