As a determiner, the word one is sometimes used before a proper noun to designate, particularly, this person: "He delivered the package to one Ronald Pepin of Colchester." The article "a" will also function in that position for the same purpose.
Sometimes we use the word one as an adjective, as in "I'll have just one scoop of ice-cream," and we seldom have trouble with that usage. But we also use one as a pronoun, and this is where one becomes surprisingly complex.
Sometimes the pronoun one functions as a numerical expression:
As a pronoun, one can also function in an impersonal, objective manner, standing for the writer or for all people who are like the writer or for the average person or for all people who belong to a class. In the United States, one sometimes has a literary or highfalutin feel to it; the more it is used, the more pretentious it feels. In British English, the use of the impersonal or generic one is more commonplace and has no such stigma. In the U.S., one is often replaced by you.
When the pronoun one is used in the numerical sense, a different pronoun can be used in a subsequent reference.
However, it is generally regarded as a bad idea to mix the impersonal or generic pronoun one with another pronoun, especially in the same sentence, as in "If one fails, then he/you must simply try harder."
In the United States, the possessive and reflexive forms of one one's and oneself are often replaced by other pronoun forms. In British English, they are commonplace:
In the U.S. that one's is apt to be replaced by a third-person "his" or (more informally) a second-person "your":
In formal writing, the use of your in that last sentence in either American or British English would be regarded as too casual or even sub-standard. On the other hand, the problem with using "his" is obvious: it runs counter to the tendency to remove gender bias from one's language as much as possible. Thus, even in American English, this mixture of "one" with "he/his/him" is slowly disappearing.*
Oneself is used in formal writing and speech as the proper reflexive form of one:
Notice there is usually no apostrophe used in the spelling of oneself. The construction one's self is used to refer to the concept of self (in psychology, for instance): "One's self, according to Freud, is defined by the interactions of the id, the ego, and the super-ego."
As a singular numerical pronoun, we don't have trouble with one: "Those donuts look delicious; I think I'll pick this one." But what if I want two donuts? It is possible, sometimes, to pluralize one:
When the word ones is preceded by a plural determiner (like these), we usually drop the ones and the determiner turns into a demonstrative pronoun: "Do you want these?"
The phrases "one in [plural number]" and "more than one" always take a singular verb:
The "one" in the phrase "more than one" apparently controls the number of the verb. It is probably wise not to attempt to divine some of the mysteries of the English language.
"One is one and all alone and ever more shall be so," goes the old Christmas song, but the fact that the singular one needs a singular verb can lead to confusion. In a recently published collection of language columns by William Safire, No Uncertain Terms, he wrote the following sentence (page 336):
"Conduct unbecoming an officer and a gentleman" is one of those phrases that sounds as if it comes out of Kipling.
The sentence caused considerable stir (as such things go), for the verb "sounds" should really relate to the plural "phrases," not the singular "one." The sentence should probably read (underlining things for our purpose):
"Conduct unbecoming an officer and a gentleman" is one of those phrases that sound as if they came out of Kipling.
The rare device for figuring out which verb to use in this construction is as follows: turn the sentence inside out:
Of those phrases that sound as if they came out of Kipling, "conduct unbecoming an officer and a gentleman" is one.
In this situation, the subject of the subordinate clause usually a who or a that will refer to the plural noun in the preceding prepositional phrase (not the one before it) and require a plural verb to follow.
There is a possible exception, however. In Burchfield's New Fowlers*, we find this example:
"Don't you think," said Bernard, "that Hawaii is one of those places that was always better in the past." (from David Lodge, 1991; my underline)
Burchfield adds, "A plural verb in the subordinate clause is recommended unless particular attention is being drawn to the uniqueness, individuality, etc., of the one in the opening clause." In an earlier note, Burchfield writes: "Exceptions [to the rule that we use the plural verb] occur when the writer or speaker presumably regards one as governing the verb in the subordinate clause," and he gives another two or three examples, including "I am one of those people who wants others to do what I think they should."
*The New Fowler's Modern English Usage edited by R.W. Burchfield. Clarendon Press: Oxford, England. 1996. Used with the permission of Oxford University Press. p. 551.