I don't understand how current reduces speed in an electric motor,while voltage increases it. Wouldn't a stronger magnetic field mean there is a stronger "force" thus should be a higher acceleration, therefore a higher speed of rotation?
Assuming you are referring to the motor slowing when you increase the field current (in a motor with a separate field winding, rather than a permanent magnet motor:
Remember what controls the speed of a lightly loaded motor : the speed increases until the back-EMF approximately equals the driving voltage. The back-EMF is generated by the motor acting as a generator. If the motor was perfectly efficient and completely unloaded, the motor would run fast enough so the back EMF equals the driving voltage; with any further increase in speed it would exceed the driving voltage and the motor would return power to the supply.
In practice the motor doesn't quite reach this ideal speed : the difference in voltage drives current through the motor's resistance, and this supplies the torque to overcome friction and the load.
Now consider what happens if you increase the FIELD current. As you expect, that increases the magnetic field. But remember the motor is acting as a generator, to generate back-EMF. Increasing the field will increase the voltage generated at any given speed : therefore the back-EMF equals the driving voltage at a lower speed. So, the motor slows down.
EDIT re : armature current. From the above explanation it should be clear that increasing armature current (by loading the motor harder) drops more voltage across the armature resistance : V=IR. Subtract this voltage from the driving voltage and you have the new, lower, back EMF : corresponding to a lower speed.