〜てしまう

Pattern: 〜てしまう
Level: N4
Reading: てしまう
Meaning: To do completely / regrettably
Attaches to: Verb (て-form)
Nuance & register: Often expresses regret, accident, or completion in spoken language.
Example: 宿題を忘れてしまいました。 I accidentally forgot my homework.
Easily confused with: 〜ておく (te oku) which means preparing in advance, whereas 〜てしまう is regret or completion.