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Difference between the indefinite article and the numeral one.
Use a/an when referring to one non-specific person or thing without emphasizing the number.
The indefinite article introduces a singular countable noun as a member of a class. The focus is on the noun itself, not on the quantity.
Use one when the number 1 is important or contrasted with another number.
One is a numeral. It is used to emphasize quantity, especially in contrast with two, three or more.
Use a/an for ordinary reference and one when you want to stress that there is only one.
Compare: "A car was parked outside." (introducing a car) and "One car was parked outside." (emphasizing that there was only one).
Use one to replace a previously mentioned singular countable noun.
One avoids repeating the same noun. It refers to a single item that has already been identified by context.
Use ones to replace a previously mentioned plural countable noun.
Ones is the plural form of the pronoun one and prevents unnecessary repetition of plural nouns.
Use one of before a plural noun to refer to a single member of a group.
Examples include one of my friends, one of the best books and one of those people. The noun after of is plural, while the verb agrees with one.
Many fixed expressions use one instead of the indefinite article.
Examples include one day, one by one, one more, one another and every one of.
Do not use one instead of a/an unless you intend to emphasize the number or use one as a pronoun.
Many learners overuse one because their native language requires an explicit numeral. In English, a/an is usually the natural choice for introducing a singular countable noun.