しまう

しまう

To finish, put away, or do something unintentionally.

Here, 'しまう' implies regret or an unintended action, emphasizing the unfortunate forgetfulness about the homework.

宿題を忘れてしまった。

I ended up forgetting my homework.

In this sentence, 'しまう' expresses an accidental or unintended outcome, specifically the breaking of the phone.

スマホを壊してしまった。

I accidentally broke my smartphone.

'しまう' here denotes the act of completing an action, suggesting a positive outcome of finishing the book.

その本を読んでしまいましょう。

Let’s finish reading that book.