We use one as a substitute for a singular countable noun and ones for a plural noun when we don’t want to be overly repetitive and when it is clear from the context what we are talking about:
- Do you need a pen? - Don’t worry, I’ve got one.
My boots are quite old, I guess it’s time I bought new ones.
1) instead of an uncountable noun:
Tell me if there’s no bread, I’ll buy some on my way back.
I thought she would buy pineapple juice, but she got orange.
2) without defining precisely which group of things we are talking about. In this case, we use some:
- I need new earrings. - Okay, let’s buy the ones with pears / the gold ones.
- I need new earrings. - Okay, let’s buy some. (not Okay, let’s buy ones)
3) after nouns used as adjectives:
I thought I only needed a birth certificate, but they said that I also needed a criminal record certificate. (not … a criminal record one)
Instead of using one / ones after possessive determiners (my, your, her, etc.) we prefer mine, yours, hers, etc. However, a possessive determiner + one / ones is often heard in informal speech:
I wish I had a family like yours.
Ones is more likely to be used in comparative sentences to refer to groups of people:
Older pupils seem to be more diligent than the younger ones.
NB! We can also use ones to refer to people in the little ones, (my, your, etc.) loved ones, the lucky ones.
- Where are the little ones? - They’re already sleeping.
1) after “which”:
When you buy sweet stuff, pay attention to which (ones) contain glucose syrup.
2) after superlatives:
Look at this tree! It’s the biggest (one) I’ve ever seen.
3) after “this”, “that”, “these” and “those”:
Help yourself to cupcakes. These (ones) are with vanilla cream and those (ones) are with chocolate chips.
4) after “either”, “neither”, “another”, “each”, “the first / second / last, etc.:
He showed me his books and said I could take either (one). (or … either of them)
- Which of these laptops is better? - I’d opt for the second (one).
1) after “the”, “the only”, “the main” and “every”:
We have many different subjects at university but I only like the ones that are in English.
2) after adjectives:
The wardrobe was too big for my room so I had to buy a small one.
NB! We can often leave out one / ones in answers after adjectives denoting colour:
- Have you decided which curtains to buy? - Yes, I’ll buy the white (ones).