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Up to now, I am still confused with this very basic grammar.

# Non-past

  * Informal : 食べる (+) and 食べない (-)
  * Formal : 食べます (+) and 食べません (-)

# Past

  * Informal : 食べた (+) and 食べなかった (-)
  * Formal : 食べました (+) and 食べませんでした (-)

Surprisingly, in order to upgrade the level of politeness (or formality), we
can only append です to the negative of the informal form.

  * Non-past: 食べないです
  * Past: 食べなかったです

The following are not allowed (I don't know why):

  * Non-past: 食べるです
  * Past: 食べたです

# Questions:

  1. Does 食べないです (and 食べなかったです) have the same level of politeness as 食べません (and 食べませんでした) has?

  2. Bonus question: Why 食べるです and 食べたです are not allowed?
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2015-12-16T12:28:35.247
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grammar
Do 食べないです and 食べなかったです have the same level of politeness as 食べません and 食べませんでした have?
1.I think so because a verbal auxiliary "です、ます" which represent formal lines
is used for them.

2.Because a verbal auxiliary "です" is set after 連体形 of the verb + の and 体言(noun
and pronoun).
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2015-12-16T13:45:11.867
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> Surprisingly, in order to upgrade the level of politeness (or formality), we
> can only append です to the negative of the informal form.

This is an interesting observation, as it is true that appending "です" to the
negative forms is common, while the same for the positive forms is highly
unusual.

The issue you observed comes from how い-adjectives are made polite in
Japanese, by appending "です" to them. For sounding very similar, and holding
some same grammatical properties as い-adjectives, ない has naturally adopted the
same grammatical rules; 痛い → 痛かった - ない → なかった, 痛いです → 痛かったです - ないです → なかったです.

Nonetheless, the foolproof way to "upgrade the level of polieteness" is, as
brought up in your post, to use the "ます"-forms. What "ないです" actually is, is a
colloquial version of the polite form (which exists both for present and past
tenses), where "です" is simply appended to the verb.

> Does 食べないです (and 食べなかったです) have the same level of politeness as 食べません (and
> 食べませんでした) has?

The level of politeness is the same, however, the main difference comes from
the fact that "ないです" is new and colloquial. Hence, you won't find it as much
in formal dialogue, as you would in informal dialogue.

It can also be argued that "ないです" is a softer expression than "ません", although
personally, I find it easy to make either expression sound soft, or hard,
depending on tone of voice.

Some might also argue that "ないです" not only is a new colloquial expression, but
grammatically ill-advised.

> Why 食べるです and 食べたです are not allowed?

They are not in use, most likely because they do not share the similarities
with い-adjectives. It is, however, possible to append "っす" (see: [What does っす
at the end of a sentence
mean?](https://japanese.stackexchange.com/questions/36/what-
does-%E3%81%A3%E3%81%99-at-the-end-of-a-sentence-mean)).
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