We use ‘one’ as a pronoun meaning ‘anyone’ to refer to people in general when we want to be formal. In everyday speech we use ‘you’ in an informal way to mean ‘anyone’.
Ex: I like to go to picnic in this area, although there are no shops here and one has to take one’s own food and drinks from home.
If you want to have a picnic here, you have to take your own food and drinks with you.
We can use an adjective on its own in a place of a singular countable noun.
Ex: Don’t buy that dress. Buy the blue one.
We can also use ‘ones’ to avoid repeating plural countable nouns.
Ex: I have new trainers, the ones over there.
We can use ‘one’ and ‘ones’ for both people and things.
Ex: Which computer are you using? – The one you gave me.
That is our new manager, the one in a blue shirt.
We use ‘one’ or ‘ones’ after ‘which’, ‘this’, ‘that’ & adjectives.
Ex: Which one do you like? This one or that one? I’d like a red one, please.
We usually avoid ‘ones’ after ‘these’ & ‘those’
Ex: I want these or I want those.
We do not use ‘one’ in a place of an uncountable noun. We repeat a noun or don’t use it at all.
Ex: That water is dirty. Take fresh water or Take fresh.