The word “one” causes difficulties only as a pronoun. A pronoun is a word which stands for a noun.

In “one of a kind” or “one at a time” or “one is good enough”, the word “one” is not functioning adjectivally: it is not describing a noun or pronoun. Rather, it functions as a pronoun: it stands for (represents or “favours”) a noun.

One use of the pronoun “one” is to abstract from the grammatical numbers, singular and plural, and the grammatical genders (feminine, masculine and neuter). In the sentence “ONE goes to Mombasa in vacations”, you are referring to no particular number or gender.

By “one”, you can mean “I”, “he”, “she, it, you, they” or, indeed, the whole society for you are generalizing.This abstraction is why you must stick to “one” whenever you use it as a pronoun in the subject.

That is, “one” must be followed by “one” — never by any other personal pronoun. You cannot say: “After one spends a week in Mombasa, he returns to Nairobi in time for him to resume work on January 2.”

For, by jumping from “one” to “he” and “him”, you are committing gross injustice to other grammatical numbers and genders. That is why you must say: “After one spends a week in Mombasa, one returns to Nairobi in time for one to resume work…”

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