Intransitive verb that becomes passive?


What are the nuances when an intransitive verbs becomes a passive verb. Is not
it just duplication:

Example

> 窓が開いた

And

> 窓が開かれた

Isnt 開かれる and 開く both mean "to be opened".




> 窓が開いた  
>  And  
>  窓が開かれた

Do you mean 窓が[開]{ひら}いた and 窓が[開]{ひら}かれた ? If so, the [開]{ひら}かれた is NOT the
passive of intransitive [開]{ひら}いた but the passive of **transitive** [開]{ひら}いた.

Active: 窓を[開]{ひら}く "(someone) opens the window" ← [開]{ひら}く is transitive  
⇨ Passive: 窓が[開]{ひら}かれる "the window is opened (by someone)"

* * *

If you mean 窓が[開]{あ}いた and 窓が[開]{あ}かれた, then your second sentence is
grammatically incorrect.

* * *

As you may know, [開]{あ}く is intransitive, [開]{ひら}く can be transitive and
intransitive, and [開]{あ}ける is transitive. To sum up:

> ◎「窓が[開]{あ}く」「窓が[開]{ひら}く」-- intransitive "the window opens"  
>  ◎「窓を[開]{ひら}く」「窓を[開]{あ}ける」-- transitive "to open the window"  
>  ◎「窓が[開]{ひら}かれる」「窓が[開]{あ}けられる」-- passive of 窓を[開]{ひら}く and 窓を[開]{あ}ける. "the
> window is opened"  
>  ✕「窓が[開]{あ}かれる」-- incorrect

Related:

  * [Distinguish between homonyms & potential form](https://japanese.stackexchange.com/q/69691/9831)
  * [ひらく / とじる vs. あける / しめる](https://japanese.stackexchange.com/q/6449/9831)

