まま and the particle に

Question

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?

Answer 1

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?

Also note that つける/つく and する/なる are transitive/intransitive pairs.

• 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。
  Someone had turned on the TV (intentionally).
• テレビがついていた。
  The TV was on.
  [neutral]