They are used to avoid unnecessary repetition. Also ONE and ONES to replace countable nouns, we can use ONE and ONES if it is clear what we are talking about.
How to use one?
1. We use "one" in countable singular nouns:
- I asked for a glass, but they did not have one.
- You can borrow a book. Which one do you want?
2. We use "one" when we don't want to repeat the name:
- Would you like a sandwich? Yes, I'd like one. (= Yes, I'd like a sandwich.)
- Is there a cashpoint near here? Yes, there's one around the corner. (= Yes, there's a cashpoint round the corner.)
3. We can use "one" alternative for this and that:
- Which cupcake would you like to have? This one or that one? (= this cake or that cake)
- Which exercise do you want to do first, this one or that one? (= this exercise or that exercise)
See: Difference Between Still and Anymore in an Easy Way
How to use ones?
1. We use "ones" with plural countable nouns:
- I broke my glasses so I will have to buy some new ones.
- There are lots of books here. Which ones are yours?
2. We use "ones" when we don't want to repeat the name:
- Would you like sandwich? Yes, I'd like ones. (= Yes, I'd like sandwichs.)
- Is there cashpoints near here? Yes, there are ones around the corner. (= Yes, there are cashpoints round the corner.)
3. We can use alternative "ones" for these and those:
- I don't know which headphones to buy, these ones or those ones. (= these headphones or those headphones)
- Which sunglasses suit me better, these ones or those ones? (= these sunglasses or those sunglasses)
Use "some and any" instead of "one and ones"
We use "some and any" with uncountable and countable nouns. You can also use "some" and "any" in a sentence without a noun if the meaning of the sentence is clear.
Examples:
- I asked for Pepsi, but they did not have any.(Pepsi)
- Fathi took lots of photos of the mountains but Aya didn't take any. (photos)
- If you need some more money, I can lend you some. (money)
- I didn't eat any salad but Adam ate some. (salad)