How can a pronoun "one" be a noun?

Question

I asked a question some days ago about if an atributive-only adjective can be followed by a pronoun one, for example in this sentence

When the Olympics began in 779 B.C. There were not a lot of events. There was only one.

...but the answers just got me more confused because I thought one, as Oxford and Longman dictionaries say, is a pronoun; but someone said it is a noun.

Could you please tell me if my sentence above is correct or not, because I think there is something wrong with that — especially with the attributive adjective only and the noun or pronoun one but I really don't know what.

Answer 1

In:

• One should be careful.

... the word one is a third person singular pronoun. Notice that even though it is singular, it doesn't need a Determiner. This pronoun one cannot be modified by adjectives and can't be postmodified by preposition phrases.

In:

• Give me one jumper.

... the numeral one is functioning as a Determiner in a noun phrase. It's a determinative according to the CaGEL. This word can also not be modified by adjectives:

• *Give me blue one jumper. (ungrammatical)

In:

• Give me the big one at the back

The word one is a bona fide noun. Notice that it can take a range of Determiners such as the, this, that, my. It also has a plural form like other nouns (and when plural can also have Determiners that occur with plural nouns such as many, more, some):

• Give me some big ones from the back.

Notice also that like other nouns it can be modified by adjectives, such as big in the example above. And like other nouns it can be postmodified by preposition phrases. So in the example above we see it modified by the preposition phrase from the back.

The OP's question

There was only one.

In older grammars this one might be regarded as a pronoun. In modern grammars such as the Cambridge Grammar of the English Language (Huddleston & Pullum 2002), this would be regarded as a determinative occurring in a fused Determiner Head construction. It is a bit like the middle example above give me one jumper but with the jumper part missing. In the Original Poster's example it means:

• There was only one [event].

The Original Poster's sentence is perfectly grammatical. We quite often use the adverb (as opposed to the adjective) only to modify noun phrases:

• It was only [a small bump].

In the Original Poster's example, one counts as a whole noun phrase.

Answer 2

"One" is usually a pronoun (or a determiner) but not always.

Answer 3

The fact that "one" can be modified by "only" is not a good reason for saying it is a noun, because "only" is not actually an adjective -- it's an adverb that can modify various things. However, it is easy to find examples in which "one" really is modified by an adjective -- "I want a blue one." Also, like a noun, "one" can be modified by a prepositional phrase. And "one" if modified can occupy the position of a noun in a noun phrase: "the blue one".

So, it's a noun. (The fact that it is sometimes called a pronoun is meaningless; that is not a fact of English.)