koto nai ことない

In Japanese, koto nai ことない means someone has "never done something," or that something has "never happened," or that something "doesn't exist," or "doesn't happen." Variants include koto ga nai ことがない and koto wa nai ことはない.

Literally, it's the combination of the light noun koto こと plus the i い adjective nai ない, "non-existent." Grammatically, koto is qualified by an adjective, such as relative clause, so it can abstractly refer to "a kind of something," and then the nai says that kind of something doesn't exist or happen.

The opposite is koto aru ことある, "exists," "happens," "I've done it."

Depending on the adjective qualifying koto, the meaning changes.

To illustrate:

• ii koto nai いいことない
  Good thing is non-existent.
  There's no good thing [about this].
  There's nothing good [about this].
  Good things never happen [to me].
• This is when an adjective word qualifies koto.
• yaru koto nai やることない
  Thing to do is non-existent.
  There's no thing to do.
  There's nothing to do.
• This is when a verb, a relative clause, qualifies koto.
• And the verb is in the non-past time.
• kanashimu koto nai 悲しむことない
  Thing to be saddened is non-existent.
  There's no thing to be saddened.
  There's nothing to be sad about.
• Phrase used when telling someone not to get sad about something that happened, because it's nothing to be sad about.
• Grammatically it's the same as the above one.
• yatta koto nai やったことない
  Thing done is non-existent.
  Doing [it] has never happened.
  [I] have never done [it].
• This is when the verb is in the past.
• This article will focus on this one.

To elaborate: if you have a noun, you can just throw ga nai がない at it, "doesn't exist," "I don't have it:"

• kane ga nai 金がない
  Money is non-existent.
  [I] don't have money.
• kanojo ga inai 彼女がいない
  Girlfriend is non-existent.
  [I] don't have a girlfriend.

But this ga が particle can only be attached to nouns, not to adjectives or verbs. Thus, the light noun koto こと acts as an adapter to make it syntactically possible to refer to adjectives and verbs.

• ureshii 嬉しい
  Happy.
• ureshikunai 嬉しくない
  Not happy.
• *ureshii ga nai 嬉しいがない
  (wrong.)
• ureshii koto ga nai 嬉しいことがない
  ureshii koto nai 嬉しいことない
  Happy stuff doesn't exist.
  There's nothing happy. Nothing happy happens.
• ureshisa ga nai 嬉しさがない
  Happiness is non-existent.
  Lacking happiness.
• Note: normally other words would be used for "happiness," this is just a grammar example.

Note that a relative clause isn't necessarily just the verb, it can have arguments:

• manga wo yonda koto nai
  漫画を読んだことない
  Thing "has read manga" is non-existent.
  I've never read manga.
• kaigai ni itta koto nai
  海外に行ったことない
  Thing "has gone overseas" is non-existent.
  I've never gone overseas.
• In Japan: "I have never visited another country." Because Japan is an island, so every other country is "overseas," kaigai.
• kanojo ga kareshi ni bentou wo tsukutta koto nai
  彼女が彼氏に弁当を作ったことない
  Thing "she made a boxed-lunch for boyfriend" is non-existent.
  She has never made a boxed-lunch for [her] boyfriend.

Furthermore, it can be in the passive:

• homerareta koto nai
  褒められたことない
  Thing "to be praised" is non-existent.
  I've never been praised [for what I do, by anyone].

Since nai ない is an adjective, it can qualify nouns itself.

• mita koto nai
  見たことない
  Thing "has seen" is non-existent.
  To have never seen [something].
  I have never seen it.
• anime wo mita koto nai
  アニメを見たこと
  Thing "has seen anime" is non-existent.
  I have never seen anime.
• anime wo mita koto nai hito
  アニメを見たことない人
  Person [that] thing "has seen anime" is non-existent.
  A person that has never seen anime.
  Someone who has never seen anime.