The reason for using 何も+negative, but 何でも+positive

Question

In one of the Japanese classes I attended, I've been taught that while we use 「何も出来ない」to say "He cannot do anything", to say "He can do anything" we use 「何でも出来る」 instead of 「何も出来る」.

Why is there a grammar rule that says 「何も」 is used before negative predicate while 「何でも」 is used before positive predicate? Why do we need additional で particle for the positive predicate?

Is the rule still being followed, and are there any exceptions (something like exceptions to the 全然+negative rule)?

Answer 1

Rather than memorizing edge cases like this one, I think the key here lies in understanding the difference between も and でも in this context.

In positive statements using も, the grouping is explicit. In other words, when you say 何も, だれも, どれも, and so on, it's clear through context or prior statements what "every" includes:

ピアノ、ギター、ドラム…彼はどれ｛○も／×でも｝上手に弾ける。 Piano, guitar, drums…he can play all of them well.

友だちはだれ｛○も／×でも｝DSを持っている。 All my friends have a DS.

私がお店に入ったとき、彼女はいつ｛○も／△でも｝いる。 When I enter the store, she's always there.

In negative statements using も, you don't have to worry about qualifying the scope of the statement, so 何もできない and だれもいない are sufficient.

With でも, however, the grouping can be left unstated:

こんな簡単な問題はだれ｛△も／○でも｝解けるよ。 Anyone can solve a simple problem like this.

これで本をいつ｛×も／○でも｝読める。 With this I can read a book at any time.

娘は好き嫌いが全然なくて、何｛×も／○でも｝食べてくれる。 My daughter isn't picky at all, and she'll eat anything.

Answer 2

This is not a very helpful answer, but: I think you are needlessly complicating a fairly simple grammar rule by looking for an "explanation". There might be some deep and obscure etymological link between the 'も' and 'でも' of 何も/誰も/いつも, but knowing it won't further your understanding of the rule itself.

It would be exactly the same as asking "why is there a grammar rule that says 'any[thing/one/time]' is used with a negative predicate while 'some[thing/one/time]' is used with a positive"...

As you can see, the analogy even holds up for the similarity between the words.

While 何でも出来ない sounds a little strange (but not unthinkable), negative forms with 何でも definitely exist. For example: 何でもないよ。 ("It's nothing").

Note: 全然+positive that you mention is not a "grammatical exception" (at least not yet). As the answers told you, it is a very colloquial use, strongly discouraged in anything more than casual conversation with friends.