～てしまう

The grammar structure ～てしまう has two main usages:

• Completion of an Action: Indicates that an action has been completed entirely.

  ほん本をぜんぶ全部よんでしまいました読んでしまいました。

  I have completely finished reading the book.

• Regret or Unintentional Action: Expresses regret, accident, or something done unintentionally, often with negative implications.

  さいふ財布をわすれてしまいました忘れてしまいました。

  I accidentally forgot my wallet.

When used in conversation, ～てしまう is often contracted to ～ちゃう in informal speech, and ～でしまう is contracted to ～じゃう.