An object will move at constant speed until acted upon by a force.
Acted on by a force.
It depends on your setting. If the net force on an object is zero than the object will move with a constant speed. It will also move with a constant speed (but not velocity!) if a force forces the object to move in a circular motion.
Accelaration
An object can move in a circle at different speeds.
Constant speed is movement at a fixed (constant) distance per unit of time. The same amount of distance is covered by the object in the same amount of time at each measurement of that same interval. An example of an object moving at a constant speed might be an object in orbit. Something up their above the drag of the atmosphere will move at a constant speed, and will continuously move around the earth at the same speed. Note that velocity is speed with a direction vector. An object in orbit will have a constant speed, but its velocity will be constantly changing because it is constantly changing the direction it is moving.
Constant speed is movement at a fixed (constant) distance per unit of time. The same amount of distance is covered by the object in the same amount of time at each measurement of that same interval. An example of an object moving at a constant speed might be an object in orbit. Something up their above the drag of the atmosphere will move at a constant speed, and will continuously move around the earth at the same speed. Note that velocity is speed with a direction vector. An object in orbit will have a constant speed, but its velocity will be constantly changing because it is constantly changing the direction it is moving.
A mass
Any moving object continues to move, in a straight line and at constant speed,when all forces are removed from it.
It's not necessary to specify that the object is moving at a constant speed. Any object with no outside forces acting on it does that. It also continues moving in a straight line.
In that case, the object is said to move at constant speed.
If the sum of all forces on an object is zero, then the object remains in constant velocity. Constant velocity means motion in a straight line at a constant speed. "Rest" is just constant velocity with a speed of zero. If the sum of all forces on an object is NOT zero, then the object's velocity must change. The change may consist of speeding up, slowing down, or turning in a different direction. If the sum of the forces is in the same direction as the object's motion, then the object must speed up.
No. If the sum of all the forces on the object is not zero, then the object's acceleration is not zero, meaning that either its speed or direction of motion, or both, are changing.