one

Sense 2a is usually a sign of a formal style. A formal style excludes the participation of the reader or hearer; thus one is used where a less formal style might address the reader directly.

• • for the consequences of such choices, one has only oneself to thank
  • —Walker Gibson

This generic one has never been common in informal use in either British or American English, and people who start sentences with one often shift to another pronoun more natural to casual discourse.

• • when one is learning the river, he is not allowed to do or think about anything else
  • —Mark Twain

Use of one to replace a first-person pronoun—sense 2b—has occasionally been criticized. It is more common in British English than in American.

• • I'm watching this pretty carefully and I hope that the issue will come up in the Lords and one may be able to speak about it
  • —Donald Coggan