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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2015-06-15T05:01:19.160
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2015-06-15T06:49:01.427
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question
grammar
～くする or ～にする versus ～くなる or ～になる
> 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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2015-06-15T06:49:01.427
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2015-06-15T06:49:01.427
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answer