N4
てしまう・ちゃう
To do something by accident, To finish completely・totally, Unfortunately
私はすぐに道に迷ってしまう。
Unfortunately, I easily get lost.
Explanation
「てしまう・ちゃう」 has 2 uses
1Doing it completely
Finishing something completely: 「宿題を全部やってしまった」 I finished all my homework.
よし!誰も食べないなら、僕が食べてしまおう。
Right! If no one's going to eat it, I'll finish it off.
今日は冬休みの宿題を全部してしまおう。
Today I'm going to get all my winter-break homework done.
2Regret, by accident
Something you didn't want happened, and you regret it: 「財布を落としてしまった」 I lost my wallet.
傘を忘れてしまった。
I forgot my umbrella.
私はすぐに道に迷ってしまう。
Unfortunately, I easily get lost.
Mixed up with
Use the context to tell 'finished' from 'regret'. If 「ワインを全部飲んでしまった」 is followed by 「美味しかった」, you simply drank it all. If it is followed by 「頭が痛い」, you drank too much and regret it.
Listening tests love the contracted forms. Know which full form matches 「〜ちゃう/〜じゃう」 and 「〜ちゃった/〜じゃった」.