[magnuz]

The starter motor is turning, but not turning the engine.
This could be the pinion not throwing forward to make contact with the flywheel, or the one way clutch unit within the pinion has failed, or teeth are missing on the flywheel or motor pinion.
Before buying a new starter, It needs to be removed so that the teeth on the fly wheel can be inspected, also check starter. if the pinion on the motor can be spun with your fingers easily in both direction, then the one way clutch has failed. If the pinion can be turned in one direction, but not the other,using your fingers then it may be ok, but could still be failing under load.
Also check the motor on the bench by securing it in a vice & connecting it to a battery, to see if the pinion throws forward along its shaft when the solenoid is activated.
Keep finger well away from pinion when doing this test !
What all this really means is .... If the flywheel teeth are ok, then you need a replacement starter motor