In English, adjectives cannot be made into nouns simply by adding an article as they can be in Romance languages.
- They have two cats: the red one is 12 years old; the other one is only 6.
Ils ont deux chats: le rouge, qui a 12 ans, et l'autre qui n'a que 6.
One is used in combination with demonstrative determiners to designate a specific object. All of these determiners can also be used as pronouns to refer more generally to an object of discourse or to uncountable nouns. Other general nouns can replace "one" , e.g. thing(s) for countables, stuff for uncountables, which are always singular.
|this one||that one|
|these (ones)||those (ones)|
One distinctive trait of some British dialects (including the standard one) is that "ones" is used with the plural demonstrative determiners these and those. This is not the case for American English, for example, which only requires the pronoun if an adjective is used (e.g. these blue ones, those broken ones)
- One guy said there wasn't going to be a break this morning.