koto aru ことある

In Japanese, koto aru ことある means something "exists" or "happens," or can happen, might happen, or that it has "happened," or that someone "has done" something before, as opposed to have never done it. Variants include koto ga aru ことがある and koto wa aru ことはある.

Literally, it's the combination of the light noun koto こと plus the verb aru ある, "to exist." Grammatically, koto is qualified by an adjective, such as relative clause, so it can abstractly refer to "a kind of something," and then the aru says that kind of something exists or happens.

The opposite is koto nai ことない: "doesn't exist," "doesn't happen," "never done it."

Depending on the adjective qualifying koto, the meaning changes.

To illustrate:

• ii koto aru いいことある
  Good thing exists.
  There is something good [about this].
  Good things happen [to me].
• This is when an adjective word qualifies koto.
• yaru koto aru やることある
  Thing to do exists.
  There's a thing to do.
  There's something to do.
  There's stuff to do.
• So don't procrastinate!
• This is when a verb, a relative clause, qualifies koto.
• yatta koto aru やったことある
  Thing done exists.
  Doing [it] has happened.
  [I] have done [it] before.
• This is when the verb is in the past.
• This article will focus on this one.

Note: although it's often translated as "I have done it before," it simply means you have done something once, or more times, as opposed to have never done it at all. Usually, "before" in Japanese is mae 前.

• mae ni yatta 前にやった
  [I've] done it before.
  [I've] done it earlier.
  [I've] done it some time ago.
• yatta koto aru やったことある
  [I've] done it before.
  [I've] done it at least once.
  [I] have experience doing it.

To elaborate the grammar: if you have a noun, you can just throw ga aru がある at it, "exists," "I have it"

• kane ga aru 金がある
  Money exists.
  [I] have money.
• kanojo ga iru 彼女がいる
  Girlfriend exists.
  [I] 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.
• *ureshii ga aru 嬉しいがある
  (wrong.)
• ureshii koto ga aru 嬉しいことがある
  ureshii koto aru 嬉しいことある
  Happy stuff exists.
  Happy things happen.
• ureshisa ga aru 嬉しさがある
  Happiness exists.
  Having 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 aru
  漫画を読んだことある
  Thing "has read manga" exists.
  I've read manga before.
• kaigai ni itta koto aru
  海外に行ったことある
  Thing "has gone overseas" exists.
  I've gone overseas before.
• In Japan: "I have visited another country before." Because Japan is an island, so every other country is "overseas," kaigai.
• kanojo ga kareshi ni bentou wo tsukutta koto aru
  彼女が彼氏に弁当を作ったことある
  Thing "she made a boxed-lunch for boyfriend" exists.
  She has made a boxed-lunch for [her] boyfriend before.

Furthermore, it can be in the passive:

• homerareta koto aru
  褒められたことある
  Thing "to be praised" exists.
  I've been praised [for what I do, by someone] before.

Since aru ある is a verb, it can qualify nouns by acting as a relative clause.

• mita koto aru
  見たことある
  Thing "has seen" exists.
  To have seen [something] before.
  I have seen it before.
• anime wo mita koto aru
  アニメを見たある
  Thing "has seen anime" exists.
  I have seen anime before.
• anime wo mita koto aru hito
  アニメを見たことある人
  Person [that] thing "has seen anime" exists.
  A person that has seen anime before.
  Someone who has seen anime before.