Conjugating present and past negative i-adjectives

Question

I've been learning about conjugating i- and na-adjectives recently and I've stumbled onto something odd. I've seen two distinct ways of conjugating negative i-adjectives; the two ways are to replace い with either:

1. くない (for present negative) or くなかった (for past negative) then add です at the end, or
2. くありません (for present negative) or くありませんでした (for past negative).

I'll use 楽しい as an example:

Present Negative:
楽しくないです。 OR　楽しくありません。==> It is not fun.

Past Negative:
楽しくなかったです。 OR　楽しくありませんでした。==> It was not fun.

Are these two algorithms for conjugating negative i-adjectives equivalent? Is one better than the other? What are any differences between them? Is one more formal than the other? Is there anything I should know about the relative usage of these two algorithms? Any input will be appreciated.

Answer 1

Both conjugations are correct, and are interchangeable in casual to moderately-formal settings. In formal written Japanese, however, ～くありません／～くありませんでした is the better choice. ～くないです／～くなかったです may be seen as colloquial, informal or even a little childish, depending on the situation.

In general, in highly stiff formal text, it's (still) safe to avoid i-adjective + です (e.g., 高いです, 高くないです, 高かったです, 高くなかったです). The use of です as a politeness marker for i-adjectives is still considered as unsophisticated and nontraditional by many, especially elderly people.

• Why should I use つかれました and not つかれたです

  です can also be a politeness marker added to adjectives. The adjective before it already inflects for tense. This is a relatively recent innovation in the Japanese language and not too long ago was considered unacceptable. Some people still try to reword things to avoid it, but it's probably caught on because it filled a useful gap in the language: making adjectives polite, like です with nouns or 〜ます with verbs, but without going as far as 〜うございます.

• correctness of い adjective + です

  い adjective + です was originally a slang used by people who were not familiar with standard Japanese and only recently (1952) officially accepted. Still now, it's not considered really sophisticated, in my opinion. Only おいしゅうございます etc were correct until then.

• Where does です come from?

There are a number of ways to rephrase non-negative i-adjective + です. For example, you can say 楽しんでいます instead of 楽しいです, 高価です or 高くなっております instead of 高いです, 赤です instead of 赤いです, 見たいと思います instead of 見たいです, and so on.

Answer 2

くない is actually く+ない and ない is a plain negative form of ある of which a polite negative form is ありません, so this is how you arrive to/from くありません. In comparison it's more formal (and possibly stiff in a more casual setting) while くない(です) is more conversational