I was thinking about this over the last few days, so I had to review a dictionary for the definitions of the word “one”.

A definition of “one” as a pronoun is an indefinite person of a kind understood by the context.

British English says that the pronoun “one” can replace the singular pronoun “I”, but American English extends this to replacing the singular pronoun “you” as well as the singular pronouns “he”, “she”, or “it”.

For example:

1. “Mum was acting strangely, and one should have noticed it.” can mean “Mum was acting strangely, and I should have noticed it.”
2. “It was as delicious as one would expect.” can mean “It was as delicious as you would expect.”
3. “After the husband and wife had driven the Ferrari, each said that it was as fast as one could desire.” can mean “After the husband and wife had driven the Ferrari, each said that it was as fast as he or she could desire.”

There you have it — a pronoun that can be a first-person pronoun, a second-person pronoun, or a third-person pronoun. No wonder English can be so difficult to learn as a second language!