まま and the particle に


> 弟はテレビをつけた **ままにしておいた**

On Dictionary of Basic Japanese Grammar the translation is **My younger
brother left the TV on.** I do not understand why にしておいた is needed. I checked
and it means : do something in advance? What do we need にしておいた for? Is it
important?

> テレビがついた **ままになっている**

Translation is **The TV was turned on and is still on** . テレビがついたまま should be
enough right? It still makes sense so why do we need the になっている phrase there?




まま and ておく play different roles, and ておく in your example has an important
meaning. See: [What's the meaning of
〜ておきます?](https://japanese.stackexchange.com/q/12964/5010)

Also note that つける/つく and する/なる are [transitive/intransitive
pairs](https://japanese.stackexchange.com/a/16322/5010).

  * テレビをつけた。  
I turned on the TV.

  * テレビをつけたままにした。  
I left the TV on.  
[maybe without purpose in particular; maybe you were just sleepy]

  * テレビをつけておいた。  
I turned on the TV and left it (in preparation / in case / for someone).  
[the TV _had been_ off; you had a clear reason to do so]

  * テレビをつけたままにしておいた。  
I left the TV on (in preparation / in case / for someone).  
[the TV _had been_ on; you had a clear reason to do so]

Compare:

  * テレビがついたままになっていた。  
テレビがついたままだった。  
The TV was still on (maybe by mistake).

  * テレビがつけ[てあった](https://japanese.stackexchange.com/q/41028/5010)。  
Someone had turned on the TV (intentionally).

  * テレビがついていた。  
The TV was on.  
[neutral]

