基本の説明Overview
関係代名詞は、2つの文をつなげて、前にある名詞（先行詞）をくわしく説明するときに使います。「〜する（ところの）…」「〜が…した（ところの）…」のように、名詞にあとから説明を足すイメージです。A relative pronoun joins two sentences and gives more detail about the noun before it (the antecedent). It adds information after the noun, like “the person who…” or “the book that I…”.
たとえば I have a friend.（私には友達がいる）と She can speak French.（その人はフランス語を話せる）を1文にするとき、2つ目の She を who に変えて、先行詞 a friend の直後に置きます。つまり関係代名詞は「代名詞（She の代わり）」と「接続詞（2文をつなぐ）」の2つの働きをもっています。説明するまとまり（who can speak French）を関係詞節といいます。To combine I have a friend. and She can speak French., change She to who and place it right after the antecedent a friend. So a relative pronoun does two jobs: it replaces a pronoun (She) and it connects the two sentences. The descriptive part (who can speak French) is called a relative clause.
どの関係代名詞を使うかは、①先行詞が人か物か、②関係詞節の中でどんな働きをするか（主格・目的格・所有格）で決まります。迷ったら、関係代名詞の直後を見ましょう。動詞が来れば主格、〈主語＋動詞〉が来れば目的格、名詞が来れば所有格（whose）です。Which one to use depends on (1) whether the antecedent is a person or a thing and (2) the role it plays inside the relative clause (subject, object or possessive). When unsure, look right after it: a verb means subject form, a subject + verb means object form, and a noun means possessive (whose).
| 先行詞Antecedent | 主格（〜が）Subject | 目的格（〜を）Object | 所有格（〜の）Possessive |
|---|---|---|---|
| 人A person | who / that | who(m) / that / （省略可） | whose |
| 物・動物A thing or animal | which / that | which / that / （省略可） | whose |
このほか、先行詞を含んで「〜するもの・こと」を表す what（= the thing(s) that）があります。また、先行詞が最上級・the only・all・every〜などをともなうときは that が好んで使われます。前置詞の直後には that を置けない、コンマをつけた補足説明（非制限用法）では that を使えない、という点も大切です。There is also what, which contains its own antecedent and means “the thing(s) that…”. When the antecedent comes with a superlative, the only, all or every…, that is preferred. Also remember that that cannot come right after a preposition, and cannot be used in a comma-separated (non-restrictive) clause.
例文で確認Example sentences
① 主格（関係代名詞が主語の働き）1. Subject form (the pronoun acts as the subject)
② 目的格（関係代名詞が目的語の働き）2. Object form (the pronoun acts as the object)
③ 所有格 whose（〜の）3. Possessive: whose
④ 前置詞とのセット4. With a preposition
⑤ 関係代名詞 what（〜するもの・こと）5. The relative pronoun what
⑥ that がよく使われる場合・コンマつきの用法6. When that is preferred / comma use
確認クイズQuick check
空所に適切な関係代名詞を入れましょう。Fill in each blank with the right relative pronoun.
- I have a friend ___ lives in Canada. (who / whom)
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who — 先行詞は人で、空所のあとに動詞（lives）が続くので主格の who。The antecedent is a person and a verb (lives) follows, so the subject form who. - This is the book ___ I read last week. (which / whose)
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which（that も可） — 先行詞は物で、あとに〈I read〉が続くので目的格。which か that で、省略もできる。The antecedent is a thing and I read follows, so the object form. which or that works, and it can be dropped. - The boy ___ mother is a nurse is my classmate.
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whose — あとに名詞（mother）が続くので所有格の whose。A noun (mother) follows, so the possessive whose. - This is the best movie ___ I have ever seen. (that / who)
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that — 先行詞が最上級のときは that がよく使われる。that is common after a superlative antecedent. - ___ I want now is a long rest.
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What — 先行詞を含んで「私が欲しいもの」を表す what。文頭なので大文字。what includes its own antecedent: “the thing I want.” It begins the sentence, so capital W. - This is the house in ___ he was born. (which / that)
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which — 前置詞（in）の直後には that を置けないので which を使う。that cannot come right after a preposition (in), so use which.
おさえておきたいポイントKey points
- 先行詞が人なら who（whom）、物なら which。that は人にも物にも使えるFor people use who (whom), for things which; that works for both
- 関係代名詞の直後を見る：動詞が続けば主格、〈主語＋動詞〉なら目的格、名詞なら whose（所有格）Look right after the pronoun: a verb means subject form, subject + verb means object form, a noun means whose (possessive)
- 目的格の関係代名詞は省略できる。主格は原則として省略できないAn object-form relative pronoun can be dropped; a subject-form one generally cannot
- 前置詞の直後には that を置けない（in which は可、in that は不可）that cannot follow a preposition (in which is fine, in that is not)
- コンマをつけた補足説明（非制限用法）では that を使わず、who / which を使うIn a comma-separated (non-restrictive) clause, use who / which, not that
- what は先行詞を含み「〜するもの・こと」を表す（the thing(s) that に言いかえられる）what includes its antecedent and means “the thing(s) that…” (it can be replaced by the thing(s) that)