One is an English language, gender-neutral, indefinite pronoun that means, roughly, "a person". For purposes of verb agreement it is a third-person singular pronoun, though it sometimes appears with first- or second-person reference. It is sometimes called an impersonal pronoun. It is more or less equivalent to the Scots "a body", the French pronoun on, the German/Scandinavian man, and the Spanish uno. It can take the possessive form one's and the reflexive form oneself, or it can adopt those forms from the generic he with his and himself.
The pronoun one often has connotations of formality, and is often avoided in favour of more colloquial alternatives such as generic you.
In Standard Modern English, pronoun one has three shapes representing five distinct word forms:
The word one developed from Old English an, itself from Proto-Germanic *ainaz, from PIE root *oi-no-, but it was not originally a pronoun. Pronoun one may have come into use as an imitation of French on beginning in the 15th century.: 224  One's self appears in the mid 1500s, and is written as one word from about 1827.
See also: Pro-form
There is a pronoun one, but there is also a noun and a determiner that are often called pronouns because they function as pro-forms. Pronoun is a category of words (a "part of speech"). A pro-form is a function of a word or phrase that stands in for (expresses the same content as) another, where the meaning is recoverable from the context. In English, pronouns mostly function as pro-forms, but there are pronouns that are not pro-forms and pro-forms that are not pronouns.: 239
|1||It depends on one's attitude.||✓||✓||"a / the person's"|
|2||I know the people who work there.||✓||✓||"the people"|
|3||Who works there?||✓|
|5||I asked her to help, and she did so right away.||✓||"helped"|
|6||JJ and Petra helped, but the others didn't.||✓||e.g., "Sho, Alana, and Ali"|
|7||Those apples look good. Can I have two small ones?||✓||"(two small) apples"|
|8||One plus one is two.|
Examples [1 & 2] show pronouns and pro-forms. In , the pronoun one "stands in" for "a / the person". In , the relative pronoun who stands in for "the people".
Examples [3 & 4] show pronouns but not pro-forms. In , the interrogative pronoun who does not stand in for anything. Similarly, in , it is a dummy pronoun, one that does not stand in for anything. No other word can function there with the same meaning; we do not say "the sky is raining" or "the weather is raining".
Examples [5–7] show pro-forms that are not pronouns. In , did so is a verb phrase, but it stands in for "helped". Similarly, in , others is a common noun, not a pronoun, but the others stands in for this list of names of the other people involved (e.g., Sho, Alana, and Ali). And in , one is a common noun. This should be clear because, unlike pronouns, it readily takes a determiner (two) and an adjective phrase modifier (small), and because its plural form is the usual -s of common nouns.: 429
Example  is a common noun. It's neither a pronoun nor a pro-form.
One can appear as a subject, object, determiner or predicative complement. The reflexive form also appears as an adjunct.
Pronouns rarely take dependents, and one is particularly resistant in this respect, though it may have some of the same kind of dependents as other noun phrases.
One generally denotes any single unidentified person, or "any person at all, including (esp. in later use) the speaker himself or herself; ‘you, or I, or anyone’; a person in general." It is usually definite but non-specific,
Monarchs, people of higher classes, and particularly Queen Elizabeth II during her reign, are often depicted as using one as a first-person pronoun. This is frequently used as a caricature by the press when they refer to the Queen or senior members of the Royal Family. For example, the headline "One is not amused" is attributed humorously to her, implicitly referencing Queen Victoria's supposed statement "We are not amused," containing instead the royal we. Another example near the end of 1992, which was a difficult year for the British royal family, as the Queen famously quipped "Annus horribilis", the tabloid newspaper The Sun published a headline, "One's Bum Year!"
In formal English, once a sentence uses the indefinite pronoun one, it must continue to use the same pronoun (or its supplementary forms one's, oneself). It is considered incorrect to replace it with another pronoun such as he or she. For example:
However, some speakers find this usage overly formal and stilted, and do replace repeated occurrences of one with a personal pronoun, most commonly the generic he:
Another reason for inserting a third-person pronoun in this way may sometimes be to underline that one is not intended to be understood as referring particularly to the listener or to the speaker. A problem with the generic he, however, is that it may not be viewed as gender-neutral; this may sometimes be avoided by using singular they instead, though some purists view this as ungrammatical (particularly when the question arises of whether its reflexive form should be themselves or themself).
Examples are also found, particularly in the spoken language, where a speaker switches mid-sentence from the use of one to the generic you (its informal equivalent, as described in the following section). This type of inconsistency is strongly criticized by language purists.
A common and less formal alternative to the indefinite pronoun one is generic you, used to mean not the listener specifically, but people in general.
When excluding oneself, one can use the generic they:
Other techniques that can be used to avoid the use of one, in contexts where it seems over-formal, include use of the passive voice, pluralizing the sentence (so as to talk about "people", for example), use of other indefinite pronouns such as someone or phrases like "a person" or "a man", and other forms of circumlocution.
Occasionally, the pronoun one as considered here may be avoided so as to avoid ambiguity with other uses of the word one. For example, in the sentence If one enters two names, one will be rejected, the second one may refer either to the person entering the names, or to one of the names.