1. 忘れ物をしました
  2. 忘れ物をしまったんです
  3. 忘れ物をしてしました
  4. 忘れ物をしてしまったんです

Are all these four expressions acceptable, what is the difference among them?
CC BY-SA 3.0
2016-06-19T03:09:58.260
36007
2016-06-20T14:02:53.237
2016-06-20T14:02:53.237
15811
question
grammar usage
Use of する and しまう
Among these, Sentences 1 and 4 are OK, but 2 and 3 are not.

  * 忘れ物 is a **noun** meaning "something someone forgot to bring", "something left behind (e.g. in a train)".
  * 忘れ物をする means "to forget to bring something", "to leave something behind (e.g., in a train)". Of course, it's a verb phrase.
  * The te-form of する is して.
  * しまう is a **subsidiary verb** which adds the nuance of "end up", "regrettably", "unfortunately" to the sentence. As a subsidiary verb, it must follow the te-form of another verb. 
    * [What is a subsidiary verb?](https://japanese.stackexchange.com/q/18952/5010)
    * [「しまう」 as an auxiliary verb](https://japanese.stackexchange.com/q/21026/5010)
  * んです and のです have various meanings, but in a sentence like this, it explains something referring to the previous sentences, putting some emphasis on the statement ("It is that ...", "I'm saying this because ..."). んです is a colloquial variation of のです. 
    * [How is the "のです" working here?](https://japanese.stackexchange.com/q/3349/5010)

* * *

Sentence 1 is a plain, matter-of-fact description ("I forgot something (e.g.,
in the room)."). Sentence 4 is correctly using both しまう and のです structures,
and sounds as if you were explaining your previous statement, with the
"regrettable" feeling.

> A: Oh, no!  
>  B: Hey, what's happened?  
>  A: 忘れ物をしてしまったんです。

Sentence 2 is incorrect because the subsidiary verb しまう is following a noun
rather than another verb. (Well, しまう can also be used [as a normal
verb](http://jisho.org/word/%E4%BB%95%E8%88%9E%E3%81%86), so actually this
sentence could mean something like "It's because I stored the lost items (eg,
in the shelf)!" But that's not what you want to say, is it?)

Sentence 3 is incorrect because する is followed by another する. Perhaps you
wanted to write 忘れ物をしてしまいました, which is probably the most common and natural
way of reporting things like this to your teacher/boss?
CC BY-SA 3.0
2016-06-19T06:53:48.043
36008
2016-06-19T06:53:48.043
2017-04-13T12:43:44.863
-1
5010
36007
answer