aratta uwagi wo hangaa ni kaketa mama heya ni oite oita.

A helpful way to read it is:

• 洗った上着を = the washed jacket / the jacket that was washed
• ハンガーにかけたまま = while still hanging on a hanger / in the state of having been hung on a hanger
• 部屋に = in the room
• 置いておいた = left there / put there and left there intentionally

So the grammar flow is:

[washed jacket] + [still on a hanger] + [in the room] + [left it there]

洗った modifies 上着.

In Japanese, a verb in plain form can directly describe a noun:

• 洗った上着 = the jacket that was washed
• more naturally in English: the washed jacket

This is a very common pattern in Japanese. English often uses that/which or a participle like washed, but Japanese just places the verb before the noun.

Because Japanese relative clauses do not use a relative pronoun.

English:

• the jacket that I washed

Japanese:

• 私が洗った上着
• or simply 洗った上着 if the subject is obvious or not important

So Japanese does not need a separate word for that/which in this kind of structure.

Because 上着 is the main thing being acted on in the sentence.

The core of the sentence is:

• 上着を置いておいた = left the jacket there

Everything in between gives extra information about the state of the jacket:

• ハンガーにかけたまま = with it still hanging on a hanger

So を marks 上着 as the direct object of the main verb 置いておいた.

At the same time, that same jacket is also understood as the thing that had been hung on the hanger.

まま means in that state, as it was, or without changing that condition.

So:

• ハンガーにかけたまま = with it still hanging on the hanger
• literally: in the state of having hung it on a hanger

The important nuance is that the state was maintained. The jacket was not taken off the hanger before being left in the room.

Because the standard grammar pattern is:

• verb past plain form + まま
• verb negative plain form + まま

Examples:

• ドアを開けたまま = leaving the door open
• 電気をつけたまま = leaving the light on
• 靴をはいたまま = still wearing shoes

So here:

• ハンガーにかけたまま = leaving it in the state of having hung it on the hanger

This is a fixed and very common pattern.

Because かける often takes に for the target or place something is hung onto.

So:

• ハンガーにかける = hang it on a hanger

The に marks the destination/target of the hanging action.

Here に marks the place where something is put/left.

• 部屋に置く = put/leave it in the room

So the two に particles are doing related but slightly different jobs:

• ハンガーに = onto the hanger
• 部屋に = in the room

〜ておく often means:

• do something in advance
• leave something as it is
• do something intentionally and let that result remain

So:

• 置いた = put/placed
• 置いておいた = put it there and left it there / left it there intentionally

In this sentence, 置いておいた suggests an intentional leaving, not just a simple momentary placement.

The most natural order is:

1. the jacket was washed
2. it was put on a hanger
3. it was left in the room in that state

So かけたまま describes the condition that continued when the speaker left it in the room.

Yes. Japanese often omits the subject when it is clear from context.

This sentence does not explicitly say who did the action. Depending on context, it could mean:

• I left the washed jacket in the room still on a hanger
• someone left the washed jacket in the room still on a hanger

Usually, a learner should assume the subject is understood from the surrounding conversation.

That is a very reasonable question.

Here, 置いておいた focuses on leaving the item in the room. The item being left is effectively the jacket together with the hanger. So even though the jacket is hanging on the hanger, the whole thing is treated as something that was left in the room.

If the speaker wanted to emphasize that the jacket itself was hung somewhere and left hanging there, a sentence with かけておいた might also be possible in a different context. But in this sentence, the focus is on the fact that it was left in the room, while still remaining on the hanger.