Grammar JLPT Level N4 - Lesson 17

By combining the conjunction particle 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.

• 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.
• 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.
• 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.
• 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

• 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.
• 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.