てしまう

This pattern has two main nuances depending on the context:

1. Completion (Aspectual): It emphasizes that an action was done to the very end. While often used in the past tense, it can also express the intent to finish something in the future (e.g., 'I will finish it by tomorrow'). It is also commonly used in commands to tell someone to finish something completely (e.g., 早く食べてしまいなさい！ 'Finish eating it quickly!').

2. Regret/Unintended Action (Modal): It expresses that something happened accidentally or that the result is unfortunate. A key element here is irreversibility—the action cannot be undone, which is why it is so common when apologizing or describing mistakes. Adverbs like うっかり (accidentally), つい, or ついつい (unintentionally) are often paired with this to signal regret. Note: While usually negative, this 'unintended' nuance can sometimes express a positive surprise or unexpected good outcome (e.g., 宝くじに当たってしまった！ 'I actually won the lottery!').

Note: These two nuances often overlap in a single sentence. For example, ケーキを全部食べてしまった can mean both 'I completely finished the cake' and 'I accidentally ate it all (and now I regret it)'. Additionally, with stative verbs (like わかる or できる), this pattern can express a complete change of state (e.g., すっかりわかってしまった 'I completely understood it') or an unexpected positive outcome (e.g., こんな難しいことができてしまった！ 'I actually managed to do such a difficult thing!'), rather than just regret.

Conjugation Rules:
Add the auxiliary verb しまう to the て form of the verb. (Note: When used as an auxiliary verb, it is almost always written in hiragana as しまう rather than the kanji 仕舞う.) The auxiliary verb しまう can be conjugated into various forms (e.g., negative, past).

• Dictionary Form: 食べる（たべる） → 食べてしまう / 飲む（のむ） → 飲んでしまう
• Polite Form: 食べる → 食べてしまいます / 飲む → 飲んでしまいます
• Polite Past: 食べる → 食べてしまいました / 飲む → 飲んでしまいました

Casual Contractions:
In daily conversation, this pattern is almost always shortened. These shortened forms conjugate just like regular う-verbs. When contracting to 〜ちゃう or 〜じゃう, any preceding small っ or ん from the て form is kept (e.g., 飲んでしまう becomes 飲んじゃう, not 飲じゃう). Warning: These contractions are strictly informal and must be avoided in formal writing or polite business contexts.

• 〜てしまう → 〜ちゃう (e.g., 食べちゃう)
• 〜でしまう → 〜じゃう (e.g., 飲んじゃう)
• Past tense: 〜ちゃった / 〜じゃった
• Volitional form: 〜ちゃおう / 〜じゃおう (e.g., 全部食べちゃおう！)
• Imperative form: 〜ちゃえ / 〜じゃえ (e.g., 食べちゃえ！)
• Rougher/Masculine: 〜てしまう → 〜ちまう / 〜でしまう → 〜じまう (e.g., やっちまった)

Negative Forms and Usage:
To express that you "ended up NOT doing" something, use ないで or ずに before しまう (e.g., 言わないでしまった / 言わずにしまった "I ended up not saying it").
Additionally, the negative conjugation of the pattern itself (〜てしまわない / 〜ちゃわない) is often used in conditionals or to express purpose/obligation. For example, 忘れてしまわないように (So that I don't end up forgetting) or 早く食べちゃわないと (I have to finish eating it quickly / If I don't finish eating it quickly).