One must be careful about his investments.

・不定代名詞"one"は、既に述べられた人や物に言及するとき、同じ名詞を繰り返すのを避けるために使われる。
・不可算名詞の代わりをつとめることはできない。
・所有格の後や基数詞の後では使えない。

[正]I like pop songs better than classical ones.
[誤]I like pop music better than classical one.
[誤]I like her songs better than his ones.
[誤]I'll have two hamburgers, and she'll have three ones.

・複数形"ones"は"these"や"those"の後では使われない。

[正]I like this one.
[誤]I like these ones.
[正]I like these.

[正]I like that one.
[誤]I like those ones.
[正]I like those.

・"one"の所有格は"one's"、目的格は"one"、再帰代名詞は"oneself"だが、堅苦しい感じがするので"he"、"his"、"him"、"himself"が使われることが多い。しかし、性差別を避けるため、"one"、"one's"、"one"、"oneself"を使う人が増える傾向にある。

[正]One must be careful about one's investments.
[正]One must be careful about his investments.

・イギリス英語においては、oneの所有格は"one’s"、目的格は"one"、再帰代名詞は"oneself"である。

[正]One must be careful about one's investments.
[誤]One must be careful about his investments.

One must be careful about his investments.の関連項目