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If they both mean the same, is が optional?

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It appears that they do have the same meaning. Maybe this is no different than any other case of particle deletion? – Nathan Ellenfield Nov 30 '11 at 4:09

気が付く has three meanings.

  1. notice, realize
  2. (often in the form よく気が付く) be attentive, be quick to notice
  3. come to oneself (usually after losing consciousness)

For senses 1 and 3, 気付く is also used. When used in these senses, I do not think that there is any difference in meaning between 気が付く and 気付く. I do not recognize any difference in formality, either, but I may be wrong.

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It surprises me to hear that 気付く is used in sense 3. I will have to keep my eyes/ears open for that one. – Hyperworm Nov 30 '11 at 14:42

As far as I understand, they mean the same thing. The one with seems slightly more formal.

Edit: Apparently this is ungrammatical, although I don't understand why. Maybe I'll make it a separate topic.

It is also more likely to be used if there is some "long" clause that modifies the . Ex:

母に電話をかけなかった気がつきました → I realized I forgot to call my mother

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Your example sentence is ungrammatical. – user458 Nov 29 '11 at 22:09
Perhaps you could improve it instead of just commenting about it. That doesn't help anyone. – istrasci Nov 29 '11 at 22:24
@sawa perhaps you could render it grammatical in the comments instead. This way we get a grammatical sentence, and you don't have to drastically affect his answer. It's win-win. – Flaw Nov 30 '11 at 3:49
As far as I can tell, 気がつく cannot directly connect to a noun or verb phrase without a particle in between it. You would something like 忘れたことに気がついた。That being said, after looking through a collection of sample sentences, both with and without が, there doesn't seem to be a distinction relating to clause length. I'm unable to offer a better explanation right now, so I'll have to defer to someone else. – Nathan Ellenfield Nov 30 '11 at 4:05
Indeed 気がつく is an idiomatic phrase as istrasci points out, but that is not the reason it cannot be modified. It simply does not make sense to do it. The meaning of relevant here is something like 'spiritual energy flow'. 母に電話をかけなかった気 would mean 'the spiritual energy flow that I did not call my mother'. – user458 Nov 30 '11 at 6:22

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