Difference between 切る and 切れる？


This question is prompted by the earlier question on the use of
[書ける](https://japanese.stackexchange.com/q/6813/921). Initially I thought the
answer might explain the difference between these two (eg 彼は頭がよく切れる| he is
(very) sharp) but in most cases, including this one the adjective seems
unnecessary. (For reference, I have made a list of common examples below.)

_(Also, as an observation, because I don't want to over-extend the scope of
the question 切れる also seems to be used in compound verbs while 切り is used in
compound "masu-形 nouns" such as 締め切り/deadline or 思い切り/to one's content.)_

* * *

**Examples of 切れる：**

> ひもが切れる| the string breaks
>
> 電池が切れる| the battery dies
>
> たばこが切れる| be out of cigarettes
>
> 賞味期限が切れる| past best before date
>
> しびれが切れる （＝足がしびれる）| legs have gone to sleep
>
> 彼は頭が（よく）切れる| he is (very) sharp

**Examples of 切る：**

> 電源を切る| switch off
>
> 野菜の水気を切る drain water from vegetables
>
> スタートを切る| start off
>
> １００メートル競走で１０秒を切る| run 100m in less than 10 secs
>
> ハンドルを右に切る| turn the wheel to the right
>
> カードをよく切る| Please shuffle the cards well. ｜
>
> キレて、犯罪を犯す若者が増加している|
>
> The number of young people committing crimes as a result of rage is
> increasing.

**Examples of compund verbs with 切れる：**

> チケットが売り切れる| tickets are sold out
>
> だれがそれを言い切れるだろうか｜ who would dare to say that with (any) certainty?
>
> 会話が 途切れる （とぎれる）｜ the conversation comes to a halt




切る and 切れる are a transitive/intransitive pair. 書く and 書ける are not.

> Is there by any chance an equally easy reason why 切れる is used in the above
> compound verbs?

Yes. Again it is about transitivity.

  * `チケットが売り切れる` \- Tickets are sold out. No overt agent is performing the action, no direct object; `売り切れる` is intransitive. Contrast against `～を売り切る`.

  * `だれがそれを言い切れるだろうか` \- Who can say/declare that certaintly? (This is not intransitive even though `切れる` is used. This should be the potential form of transitive `言い切る`.)

  * `会話が途切れる` \- The conversation comes to a halt. No overt agent is performing the action, no direct object; `途切れる` is intransitive. 




切る and 切れる are a transitive/intransitive pair. 書く and 書ける are not.

> Is there by any chance an equally easy reason why 切れる is used in the above
> compound verbs?

Yes. Again it is about transitivity.

  * `チケットが売り切れる` \- Tickets are sold out. No overt agent is performing the action, no direct object; `売り切れる` is intransitive. Contrast against `～を売り切る`.

  * `だれがそれを言い切れるだろうか` \- Who can say/declare that certaintly? (This is not intransitive even though `切れる` is used. This should be the potential form of transitive `言い切る`.)

  * `会話が途切れる` \- The conversation comes to a halt. No overt agent is performing the action, no direct object; `途切れる` is intransitive. 

