grammar - ことができる versus V~える form

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?

Answer

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.