In the JLPTN5, 5th question, "弟は部屋◯掃除をしました"

I'm trying to understand why the の option is more appropriate than the に
option. Both appear to be correct in English.

With に we are taking "the room" as indirect object (since に can be a
substitution for the へ particle) and "cleaning" as a direct object; on the
other hand, with the の option, we are taking "room's cleaning" as a direct
object only. Am I forgetting something important here? Thanks for help.
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grammar jlpt
Jlpt/n5q5: 弟は部屋◯掃除をしました。◯: の versus に option
弟｛おとうと｝は部屋｛へや｝の掃除｛そうじ｝をしました。

With の it is saying the younger brother did the cleaning of the room. This
clearly states with no uncertain terms that he cleaned _the room_.

弟は部屋に掃除をしました。

The latter isn't as clear cut from what I can tell.

I originally thought that it would end up meaning that he was cleaning
something in the room, but as Earthliŋ said, since cleaning is an activity,
the correct particle for saying that he was cleaning _something_ in the room
would be で: 「弟は部屋で何かの掃除をしました。」.
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It is because there is a を after 掃除.

If the verb of a sentence is a "suru verb", you can split apart the verb (掃除
in this case) and する. If you do, 掃除 will be interpreted as a noun and will be
the direct object of the sentence's main verb する.

> 掃除する - To clean  
>  掃除をする - To do cleaning

Are both valid phrases- but in the former, the entire thing is interpreted as
one long verb, and in the latter the verb is only する and 掃除 is the direct
object of suru. In other words, 掃除 is the thing you are doing.

Doing this, though, introduces a problem. If there is already a direct object
in the original sentence, there will now be two を in the sentence. To avoid
confusion, the original direct object (部屋 in your example) is marked with の
instead.

> 部屋を掃除する - To clean the room  
>  部屋の掃除をする - To do the room's cleaning

They are both essentially the same, but if you are interested in differences
you can look at [this
question](https://japanese.stackexchange.com/q/4006/9749).
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## 掃除する

The phrase 掃除する acts like a single verb. It's technically made of two words:

  1. the verbal noun 掃除
  2. the verb する 

But together they act like a single verb. In this case, that verb is
transitive, which means it takes a direct object marked by を:

> 部屋をobject 掃除するverb 'clean the room'

The verb is 掃除する, and its direct object is 部屋.

* * *

## 掃除をする

When you put を between 掃除 and する, they no longer act together like a single
verb:

> 掃除をobject するverb 'do cleaning'

Now the verb is する by itself, and its direct object is the regular noun 掃除.

How do we add 部屋 to this clause? Well, the verb する already has one direct
object, and we can't add another:

> ＊部屋 **を** object 掃除 **を** object するverb ← ungrammatical

But we don't really want to relate 部屋 to する anyway. We want to relate it to
掃除, and that's a noun. How do we show a relationship between two nouns in
Japanese? With the particle の:

> [ **部屋の** 掃除 ]-を する 'do [the cleaning **of the room** ]'

Now the verb is する 'do' and the direct object is the entire phrase 部屋の掃除 'the
cleaning of the room'.

* * *

## Notes

  1. Sometimes people explain 掃除する as ellipsis of 掃除をする. But it's important to realize that this is not the case:

> ​ 部屋 **を** 掃除 **** する ← OK  
>  ＊部屋 **を** 掃除 **を** する ← ungrammatical

Adding を in like this makes it ungrammatical, so it can't be ellipsis.

  2. In this answer, I used English translations like 'do the cleaning'. In fact, you'd usually translate both 掃除する and 掃除をする to 'clean'. That's because when you translate, you look for the most natural way to express something in the target language. But in this case, my goal was to try to show the grammar of the original Japanese, so I deliberately used less natural sounding translations.

  3. In linguistics, phrases like 掃除する are called 'incorporated', while phrases like 掃除をする are called 'unincorporated'.

  4. The giant asterisk ＊ is used in this answer to mark a sentence as ungrammatical.
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