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kinou ha saihu wo nakusimasita.

昨日きのう　は　財布さいふ　を　無なくしました。kinou ha saihu wo nakusimasita.
I lost my wallet yesterday.
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Questions & Answers about kinou ha saihu wo nakusimasita.

Why is は used after 昨日 instead of the particle に?

In Japanese, は can mark a time expression as the topic—“as for yesterday…”—which often gives a slight contrast or emphasis on that time. Using に (昨日に) would simply state “at yesterday,” focusing purely on when something happened. By saying 昨日は, you’re highlighting yesterday itself as the frame for the rest of your comment (“Yesterday, (I) lost my wallet”).

Why is there no subject (like “I”) in this sentence?

Japanese frequently omits pronouns when the subject is clear from context. Here, the speaker is obviously talking about themselves, so 私は (“I”) is dropped. Adding 私は isn’t wrong, but it’s redundant if you already know who’s speaking.

Why is 財布 followed by を in 財布を無くしました?

The particle を marks the direct object of a transitive verb. Since 無くしました (lost) is a transitive action—the speaker actively lost their wallet—they must use を to indicate “wallet” is what was lost.

What’s the difference between 無くしました and なくなりました?

They look similar but differ in transitivity:

  • 無くしました is the past of the transitive verb 無くす (“to lose something”), implying someone lost it.
  • なくなりました is the past of the intransitive なくなる (“to become lost/disappear”), focusing on the thing going missing on its own.
    You’d use 無くしました when you’re the one who lost it, and なくなりました if you want to say “It got lost.”
Why is the verb in the ～ました form here? Is this polite speech?

Yes. The ～ました ending is the polite past tense in Japanese (the “-do/desu” style). 無くしました is more polite than the plain past なくした, making the sentence appropriate for everyday conversation with someone you’re not extremely close to or when you want to be a bit more formal.

I see 無くしました written with the kanji 無くす, but could I write it in hiragana or use a different kanji?

Absolutely. Verbs like なくす can be written in:

  • Full hiragana: なくしました
  • Kanji + hiragana: 無くしました or 失くしました
    The choice is mostly stylistic or based on readability. All three forms are acceptable, though some publishers prefer 失くす for “losing a tangible object” and 無くす for more abstract losses.
What’s the difference between 無くす and 忘れる, since both can involve “losing” something?

They’re different verbs:

  • 無くす means “to lose” in the sense of misplacing an object (wallet, keys, phone).
  • 忘れる means “to forget” in the mental sense (forgetting an appointment, someone’s name, or to do something).
    You use 無くす when the thing itself disappears from your possession, and 忘れる when you fail to remember.