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Way back in the day when I was first learning Japanese, I learned that you could add ことができる to a verb to indicate potential. Like so:

食{た}べることができる

(I) can eat (something)

It became my habitual way of expressing possibilities.

Then later, I learned that you could just modify the verb and get the same thing:

食{た}べられる

(I) can eat (something)

I know that this is also the passive voice, but, unless I'm mistaken, it can be used strictly as a way of expressing potential.

I still tend to habitually say ことができる, though. It tends to jump out of my mouth before I realize that I could have probably modified the verb. I think because my brain thinks they are the same thing, so I just go with the usual.

So... are they different?

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The question has two occurrences of ことができない, but did you mean ことができる? Obviously ことができない is the negation of ことができる. –  Tsuyoshi Ito Aug 17 '11 at 3:53
    
@Tsuyoshi Ito: They were typos. Corrected now. Thanks for pointing it out. –  Questioner Aug 17 '11 at 3:55
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<sarcasm>Without using ことができる, you can't build awesome sentences such as 「意地になって争って秘孔の事を刺すことを入れることをすることにふれることができません」</sarcasm> –  syockit Aug 20 '11 at 3:40

3 Answers 3

up vote 15 down vote accepted

Very simply :

  • 食べることができる

I am technically able to eat. I have a mouth, a stomach, and so on. When you ask "can you do this for me" and your witty friend replies "yes, I can" but doesn't do it, that's this meaning of potentiality that he chose to understand. You'd use this form to say "I cannot time travel" or "I cannot fly". You cannot do anything about it, you're not responsible (which is a very Japanese way to say things).

  • 食べられる

I can eat, in the other meanings :) For example, "I can't go with you because I have some work to finish". Technically, you can go, but for some reason, you must abstain from going. You can do something about it, you're responsible for not going/doing.

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6  
This fits very well with the answer I was thinking of writing. I think the difference shows up even more clearly with the negatives, which you touched on a bit. 食べることができない: I am physically incapable of eating; there is something (such as a medical condition or apparatus) preventing me from eating. 食べられない: I am physically capable of eating, but for other reasons (food preferences, religious reasons, time constraints) I cannot eat. –  Derek Schaab Aug 18 '11 at 12:32
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I like this answer too. It seems to fit with what I've been told by some Japanese people. I like to translate 〜ことができる using the English "able", much like Axioplase. "食べることができますか?" == "Are you able to eat?". Contrast that with 食べられますか? "Can you eat?" and I think it demonstrates the difference nicely. –  phirru Aug 18 '11 at 12:53

I think ~ことができる has more to do with the potentiality (real word?) of things out of your control and ~(ら)れる has more to do with your abilities or things you can control.

  • 雨が止んだら、テニスにいくことができるよ! → After the rain stops, we'll be able to go play tennis (can't control the rain).
  • こつこつ日本語勉強するなら、難しい漢字も読めるようになる。 → If you keep up with your Japanese studies, you'll be able to read even difficult kanji. (can control this)

Of course, this is a slight nuance, and I think the two forms overlap a great amount.

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Interesting. I'd say the exact opposite, partly because 〜れる always sounds a bit passive to me. –  Zhen Lin Aug 17 '11 at 5:52
    
this makes sense to me cause i was under the impression that you couldn't use the potential form with intransitive verbs because they fall into this special group verbs (begins with 無 i think, sorry for being of little help here). So in that case i think you could get away with the ことができる –  Mark Hosang Aug 17 '11 at 6:40
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@Mark 行く is intransitive yet there is for sure the potential form of 行ける & negative 行けない. –  Lukman Aug 17 '11 at 6:51
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@Mark I think you are referring to the stative verbs, which are those that describe states rather than actions. Examples of them are 聞こえる and 分かる. –  Lukman Aug 17 '11 at 7:07
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@Mark 「入る」is kind of tricky, it can be 無意志動詞 or 意志動詞 according to the subject. E.g. 「私は男性だから、入りたくても女子トイレに入れない」 is correct, so one might be inclined to say 「タンスが大きすぎて部屋に入れない」 even though the latter is incorrect. –  syockit Aug 18 '11 at 8:01

I'm not entirely sure, but ことができる I think is more 固い表現 than using the potential verb. And is usually taught before the potential form because grammatically structures are easier for students to pick up.

here is a forum with the same exact question btw

http://forum.wordreference.com/showthread.php?t=135581

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If you mean “formal expression,” it should be 固い表現 or 硬い表現. But formality is different from politeness. –  Tsuyoshi Ito Aug 17 '11 at 12:49
    
thanx ito, edited my post. and you are right that politeness and formality are different. –  Mark Hosang Aug 18 '11 at 8:44

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