By combining the 接続助詞（せつぞくじょし）
て, with the verb
しまう (sometimes written as
終（しま）う), you are able to express that something was done ‘by accident’, ‘completely’, or ‘unfortunately’. These meanings may seem quite different, but we will discuss their similarities in a moment. Let’s look at a few examples of
てしまう first.
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私（わたし）はすぐに道（みち）に迷（まよ）ってしまう。
Unfortunately, I easily get lost.
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家（いえ）にお菓子（かし）がいっぱいあると全部（ぜんぶ）食（た）べてしまうのでお菓子（かし）を買（か）わないようにしている。
When there are a lot of snacks in my house, I completely eat them all, so I try not to buy them.
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メニューにアイスクリームがあると頼（たの）んでしまう。
When there is ice-cream on the menu, I unfortunately order it.
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私（わたし）はよく舌（した）を噛（か）んでしまう。
I accidentally bite my tongue often.
As we can see, verbs that usually take
で instead of
て will be no different when using
てしまう.
Sometimes,
てしまう or
でしまう will be shortened to a more casual form, which are often used in speech. These forms are as follows:
てしまう will become
ちゃう
でしまう will become
じゃう
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最近（さいきん）運動（うんどう）をしてないから太（ふと）っちゃう。
Recently, I haven't been exercising, so I’m totally going to get fat.
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花（はな）が死（し）んじゃうから、花（はな）を踏（ふ）むな。
Since the flowers will unfortunately die, don’t step on them.
While
て終（しま）う is most frequently written in hiragana,
て仕舞（しま）う may also be seen. There is a slight nuance difference between these two kanji structures, but the primary meaning of
しまう will always be that ‘something has come to the point where ending/ending up in a specific state cannot be avoided’. This ‘no turning back point’ may be temporary (likely to be written as
仕舞（しま）う), or it may be permanent (likely to be written as
終（しま）う).
Due to this ‘cannot be avoided’ nuance,
しまう often comes across as ‘totally’, as in ‘ah… I totally left the chicken in the oven for too long’. In these cases, it is simply expressing the speaker's exasperation that something is ‘too late’ to change. However, despite regularly being used negatively,
てしまう can also be used positively. In these cases it still has the nuance of ‘no turning back’.
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ゲームを始（はじ）める前（まえ）に宿題（しゅくだい）を終（お）わらせてしまおう！
Let's completely finish my homework before I start playing video games! (I'm going to completely finish my homework before I start playing video games)
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青木（あおき）先輩（せんぱい）、定時（ていじ）までに社長（しゃちょう）に頼（たの）まれたレポートを完成（かんせい）させてしまいましょうよ。
Aoki senpai, let's completely finish the report that our boss has asked us to do before work is over.