Difference Between "何も" and "何でも"


What is the difference between "何も" and "何でも"?

I have commonly heard that "何も" means "everything/nothing" and "何で" means
"anything" until my Japanese teacher (a native speaker) told me that the
difference between these two is that "何も" is only used in negative sentences
and "なんでも" only in positive sentences.

But then both "何でもいい" and "何でもない" seem to be common grammatically-correct
expressions. One is positive and the other is negative.

Could someone please clarify what the actual difference between "何も" and "何でも"
is?




As your teacher say, the opposite meaning of ''なんでも食べる'' is ''なにも食べない'', the
opposite of ''なんでもある'' is ''なにもない''.  
Now what is ''なんでもない''!?  
  
The expression of its opposite meaning is なにかだ or なにかである, though that is
rarely used.  
  
ない in the sentence ''なんでもない'' is not _not to exist_ but just a negative form
of 〜だ or 〜である. You can also say ''なんでもありません'' with politeness.  
  

> A. この店にはなんでもある。 (This shop has everything.)
>
> B. この店にはなにもない。 (This shop doesn't have anything.)  
>

A and B are spoken about what the shop has. ある and ない in these sentences have
the meanings of _to exist or not._  
  

When can we use ''なんでもない''?  

If your friend looks disappointed and blue, you would ask him or her what
happened and say ''どうしたの？''.  
S/he can answer ''なんでもない'' if s/he doesn't mention what happened on him or
her, or s/he doesn't want to make you worry.  

* * *

  

(added)  
  
I want you to think about ''だれもいない'' and ''だれでもない'' too.  
  
''だれもいない'' is _Nobody exists._ or _There is nobody._ It's the same usage of
''なにもない''.  
While, ''だれでもない'' is _It's not anyone._ This is the same usage as ''なんでもない''.  
Please notice that not いない but ない is used in ''だれでもない'' even though ''いる/いない''
(not ある/ない) should be used for persons. That's because the ない in ''だれでもない'' is
not _not to exist_ but just _not_ , is the same as ない in
''私{わたし}は学生{がくせい}ではない''.  
So both ''なにもない'' and ''なんでもない'' are grammatical.  
  

  * なにかがある。Something exists.
  * **なにもない** 。Nothing exists.
  * なにかだ。(the same as なにかである) It's something.
  * **なんでもない** 。It's not anything.   
  

  * だれかがいる。Someone exists.
  * だれもいない。Nobody exists.
  * だれかだ。It's someone.
  * だれでもない。It's not anyone.

