Learn Hardcore Japanese: watasi ha ukkari ie ni siryou wo wasurete simaimasita.

bold うっかり means “carelessly,” “inadvertently,” or “by mistake,” highlighting that the action was unintentional. It functions as an adverb and typically goes right before the verb phrase: bold うっかり忘れる. Variants you’ll also hear are bold うっかりして忘れる and (less commonly) bold うっかりと忘れる.

Why is bold に used after bold 家 instead of bold で?

With bold 忘れる (to forget/leave behind), bold に marks the place where the item ends up being left. So bold 家に資料を忘れる means “to leave the materials at home.” bold 家で would mark the place where an action occurs; using bold 家で here sounds off if you mean “left them at home.” Use bold に for this pattern.

What does bold 〜てしまいました add to bold 忘れて?

The construction bold Vてしまう can show completion (“finish doing”) or an unintended/regrettable result. With bold 忘れる, it almost always adds the “unfortunately/accidentally ended up …” nuance. So bold 忘れてしまいました softens the statement and hints at regret or apology compared to plain bold 忘れました.

How would this sound in casual speech?

Common casual versions: bold うっかり家に資料を忘れちゃった or simply bold 家に資料を忘れちゃった. The contraction is bold 〜てしまう → 〜ちゃう (and bold 〜でしまう → 〜じゃう). You might also hear the interjection bold しまった! (“Oh no!”) on its own.

Is bold 私 necessary? What about bold 私が?

Subjects are usually dropped when obvious, so native speech would likely omit bold 私. Using bold 私は sets “me” as the topic; using bold 私が emphasizes contrast/focus (“It was I who …”), often when clarifying responsibility.

Can I change the word order?

Yes. Japanese allows flexibility before the verb. Besides bold 家に資料を忘れてしまいました, you can say bold 資料を家に忘れてしまいました. The original order is very common for this “leave X at place” pattern.

What’s the difference between bold 家, bold うち, and bold 自宅 here?

bold 家: neutral “house/home,” widely usable.
bold うち: “my place/home,” casual and personal.
bold 自宅: “one’s residence,” more formal or businesslike.
All can fit; pick based on tone: bold うち (casual), bold 家 (neutral), bold 自宅 (formal).

How do I say “I forgot to bring the materials,” not just “left them at home”?

Use the “forget to do” pattern: bold 資料を持ってくるのを忘れてしまいました. More formal: bold 資料を持参するのを忘れてしまいました. These focus on the intended action you forgot, not the location.

Why is bold 資料を used instead of bold 資料は? When would bold は be okay?

bold を marks bold 資料 as the direct object of bold 忘れる. Using bold は topicalizes/contrasts: bold 資料は忘れてしまいました (implying “but other things I didn’t”). Use bold は when you want that contrastive emphasis.

Is bold 忘れる transitive, and how is bold 忘れて formed?

bold 忘れる is a transitive ichidan (る) verb: you forget something (bold 資料を忘れる). The て-form is made by dropping bold る and adding bold て → bold 忘れて.

Why is bold しまいました written in kana?

As an auxiliary (bold 〜てしまう), it’s generally written in kana in modern Japanese. The standalone verb “to put away/finish,” bold 仕舞う, is sometimes written with kanji, but the auxiliary is usually kana-only: bold 〜てしまう / 〜てしまいました.

Can I use bold 置いてきた or bold 置き忘れた instead of bold 忘れた?

Yes, with nuance changes. bold 置いてきた emphasizes physically “left it behind and came (here).” bold 置き忘れた means “left it behind by forgetting.” You can combine with regret: bold 家に資料を置いてきちゃった / 置き忘れてしまいました.

Is bold 家で資料を忘れました really wrong?

For “I left the materials at home,” natives say bold 家に資料を忘れました. bold 家で資料を忘れました can sound unnatural or be misread as describing the location of the forgetting event rather than the item being left behind. Use bold 〜に忘れる for the “left at [place]” meaning.

Can particles be dropped here in casual talk?

In very casual speech, some particles may be omitted: bold 家に資料、忘れちゃった. However, for clarity and correctness—especially as a learner—keep particles: bold 家に資料を忘れちゃった.

What’s the difference among bold うっかり, bold つい, and bold 思わず?

bold うっかり: carelessly/inattention (fits forgetting well).
bold つい: “without thinking/just (ended up),” often with actions you slipped into (bold つい食べちゃった).
bold 思わず: “unconsciously/instinctively,” often for spontaneous reactions (bold 思わず笑った).
All can pair with bold 〜てしまう, but bold うっかり is the most natural with bold 忘れる.

How could I phrase a more formal apology?

Avoid bold うっかり (it can sound too casual/blunt about carelessness). Use something like: bold 資料を自宅に置き忘れてしまい、誠に申し訳ございません. You can also say bold 資料を持参できず、申し訳ございません.

Does bold 資料 mean one document or several?

Japanese doesn’t mark plural by default. bold 資料 can mean “material(s)/document(s)” depending on context. If needed, you can add quantity (e.g., bold 資料を三部) or use bold 諸資料 (“various materials”) to make plurality explicit.