性別が分からない・特定しない1人を受ける代名詞は、現代英語では they が標準です: Someone left their umbrella.(誰かが傘を忘れていった)。動詞は複数形のまま(they are)。古い his or her より自然で、主要な辞書・スタイルガイドも公認しています。
For one person whose gender is unknown or unspecified, modern English standardly uses they: Someone left their umbrella. The verb stays plural in form (they are). It reads more naturally than the old his or her, and major dictionaries and style guides have signed off.
「誰か」を受ける自然な形
The Natural Way to Say "That Person"
If anyone calls, tell them I'll be back at three.(誰か電話してきたら、3時に戻ると伝えて)。この them は1人の「誰か」を受けています。かつての規範は his(男性形で代表)や his or her(併記)でしたが、前者は偏り、後者は冗長。単数の they はこの両方を解決する形として定着しました。実は新しい発明ではなく、チョーサーやシェイクスピア以来の長い歴史を持つ用法の復権です。
If anyone calls, tell them I'll be back at three — that them stands for one unknown caller. The old prescriptions were his (male-as-default, biased) or his or her (accurate, clunky). Singular they solves both problems, and it's no modern invention: the usage runs back through Chaucer and Shakespeare. What we're seeing is a restoration, not a novelty.
単数の they の現場Singular they at work
Someone left their phone on theなぜ the なのか【決まった場所・乗り物】the station / the bus のように、話し手と聞き手の生活で「あの一つ」に決まる施設や便を指す。 train.
every(単数)を受けるのも their が現代標準。Even after singular every, their is the modern standard.
Ask theなぜ the なのか【前に付く語で一つに決まる】top・right・whole・next・same のように、前に付く語がそれ自体で対象を一つに絞るので the。後ろに限定する句は無い。 next customer what they would like.
次のお客様に、何になさるか伺って。
特定の1人でも性別を持ち出さずに受けられる。A specific person, referred to without invoking gender.
they(現代標準)
自然・簡潔・偏りなし。話し言葉でも書き言葉でも。Natural, compact, unbiased — speech and writing alike.
Somebody's at the door — tell them to wait.
誰か来た。待ってもらって。
his or her(旧規範)
正確だが冗長。今は形式ばった文書でも減少中。Accurate but clunky — fading even from formal prose.
Each applicant must submit his or her form.
応募者は各自の書類を提出のこと。(古風な書き方)
よくある間違い / Common Mistake
×Someone called you. He left a message.
誰かから電話があったよ。伝言を残していった。(相手の性別を知らずに)
○Someone called you. They left a message.
誰かから電話があったよ。伝言を残していった。
性別が分からない相手を he で受けるのは、現代では事実の誤りに聞こえます(なぜ男性と決めつける?)。分からないときは they。これは文法の正誤を超えて、聞き手への配慮の標準になっています。
Defaulting to he for an unknown caller now sounds factually off — why assume? When you don't know, it's they. This has moved beyond grammar into the standard of consideration.
their own ...各自の〜everyone / each と組んで「一人ひとり自分の」を言う定番。Pairs with everyone and each for "each their own."
if anyone ..., they ...誰かが〜したら、その人は…anyone を受ける標準の型。he or she より自然。The standard frame after anyone — smoother than he or she.
tell them / ask themその人に伝えて/聞いて正体不明の1人への指示は them 一択。Instructions about an unknown someone run on them.
to each their own人それぞれ単数の they が慣用句にまで定着している証拠。Singular they, fossilized into idiom — proof of tenure.
まとめ / Summary
性別不明・不特定の1人= they / their / them。Unknown or unspecified person: they, their, them.
動詞は複数形のまま(they are)。The verb keeps its plural form: they are.
his or her は冗長な旧規範。His or her is the clunky old rule.
本人の代名詞としての they も辞書公認。相手の希望に合わせる。Personal they is dictionary-recognized — follow people's stated pronouns.