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My Japanese text book says that a present-tense verb followed by ことにする is used to indicate making a decision about the action. But I've also noticed the verb 決める, and that it's usage is very similar.

So supposing we have a sentence such as:

朝ご飯を作った後に、ジョギングすることにした。(After making breakfast, I decided to jog.)

My question is, would there be any specific semantic difference if 決める was used? In all the cases that I've seen, the final する has always been in the past-tense, but 決める has been in a variety of forms; is this possible difference of the two?

Thanks! :)

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3 Answers 3

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This is what "A Dictionary of Basic Japanese Grammar" has to say on the matter of ことにする vs ことに決める。

"Koto ni suru and koto ni kimeru 'determine to do s.t.' are virtually identical in meaning. The difference is that the former is an idiom and, therefore, frequently used in colloquial speech, while the latter is appropriate when the speaker is talking about a relatively important decision in a rather decisive manner. Also, koto ni suru can be used to mean 'I hereby decide to ~' but koto ni kimeru cannot. Thus, [1] below cannot be rephrased by koto ni kimeru.

[1] 私は会社をやめることにします。- watashi wa kaisha wo yameru koto ni shimasu. - I've decided to quit my company."

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As far as past/present tense goes, I doubt there's any difference between the two. It just seems kind of weird to say "Now that I've finished breakfast, I will decide to go jogging." OTOH, if it's something more important, "I will take a decision tomorrow" is OK. –  rdb Aug 21 '11 at 6:07

Using your example sentence as a guideline, the difference is this:

With ことにした, you have decided to jog, and you might have jogged. ことにする implies action taken. Usually in the sense that you habitually do it. However, you might just mean you've made a decision.

With ことに決めた, you have decided to jog, but whether or not you have jogged yet is an open question. You have only conveyed your intentions. It strictly only refers to a decision with no implication of action.

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I don't think this is completely correct. ことにした is referring to a decision being made, not the action being completed. For example: 夏は北海道を旅行することにした. This means you have decided that you will travel around Hokkaido in summer, not that it has been completed. –  phirru Aug 21 '11 at 6:01
    
I can believe that I might be wrong on this. However, note that I'm not saying that ことにした means one has necessarily completed the action. It is, after all, as you say in your answer an idiom that does largely the same job as 決める. I am just saying that there is an implication of having also actually done the action. So ことにする can mean decided and have done or just decuded, whereas 決める only means have decided. I'll edit my answer to make that more clear. –  Dave M G Aug 21 '11 at 6:50
    
Oh okay, yeah that makes perfect sense. I originally took your answer to mean the decision AND action have both been completed. –  phirru Aug 21 '11 at 7:08

ことにする vs ことになる is equal ことに決めた

私は会社をやめることにします。(自分が決めた) I've decided to quit my company. 私は入院中はタバコをやめることに決めた。(他人(医者が決めた))is equal 私は入院中はタバコをやめることになった。(他人(医者が決めた)) I had to abstain from smoking while I was in the hospital.

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This is too sketchy to be an answer to begin with, but one does not say 私は入院中はタバコをやめることに決めた if it is decided by a doctor. –  Tsuyoshi Ito Aug 22 '11 at 22:54

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