According my teacher,
- ～くする or ～にする is used for direct action, but
- ～くなる or ～になる is used for indirect action
For me, the meaning of direct and indirect is a bit ambiguous. Consider there is a tool than can cut something. The tool can be either powered by a motor or powered by human energy. Is the decision whether directly or indirectly based on the source of energy to do the action?
I mean the sentence A below will be true if the tool is powered by a motor, for example. Otherwise it will be false if powered by a human.
With almost the same logic, the sentence B below will be true if the tool is powered by a human. Otherwise it will be false if powered by a motor.
What do you think? More precisely, when do we have to use する and なる? Any comment is appreciated.