One may be used in the nominative, but (much unlike French on and German man) it can also be used in other cases. It occurs most commonly in sentences in the present simple tense or conditional constructions. Examples of its use:
There is no strong form analogous to hers and yours: *One's is broken; *I sat on one's; *I broke one's.
Oneself is anomalous in its inability to refer back to anything other than one:
Some consider one to be overly formal, and avoid it. However, in doing so, they encounter problems only resolvable by awkward phrasings or a significant drop in formality. In particular, phrasing a sentence in a gender neutral way may require the passive voice, singular they, pluralizing, you, or circumlocution. In addition, the word one can also be used for inanimate objects, creating possible confusion in careless writing. For example,
The second one may co-refer with the first, or it may refer to a specific rule. (If this sentence were spoken at all, the second one would require distinctive intonation for the second interpretation.)
One may have come into use as an imitation of French on. French on derives from Latin homo, nominative singular for human. It is distinct from the French word for the English numeral one un(e), which never appears as a pronoun.