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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.

A: このツールを使うと、ズボンが短くなります。

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.

B: このツールを使うと、ズボンを短くします。

What do you think? More precisely, when do we have to use する and なる? Any comment is appreciated.

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  • I'm not quite sure why the motor is important. In either situation the machine is still operated by a person and I'd say the same rules apply to each situation.
    – ssb
    Jun 15, 2015 at 5:06
  • 2
    このツールを使って、ズボンを短くします。
    – Yang Muye
    Jun 15, 2015 at 5:17
  • @ssb: Let's compare scissors and an automatic cutting machine. Jun 15, 2015 at 5:18
  • このツールを使うと、ズボンを短くします sounds unnatural.
    – Chocolate
    Jun 15, 2015 at 6:54

1 Answer 1

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A: このツールを使うと、ズボンが短くなります。

If you use ~くなる or ~になる here, you intend to mean this: "If you use this tool, the pants becomes smaller."

B: このツールを使うと、ズボンを短くします。

If you use ~くする or ~にする here, you intend to mean this: "If you use this tool, you make the pants smaller."

As opposed to ~くなる or ~になる, ~くする or ~にする implies that you have made a decision or have made an action yourself.

~くなる or ~になる on the other hand, you're just making a statement or an observation that something has undergone change.

Hope this helps.

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