One

We commonly use one and its plural ones as a substitute for a countable noun:

A:
What about a cake?

B:
All right. Thank you. But I’m trying to slim, so I’ll just have a smallone.

A:
Can you fetch the DVDs from the shop?

B:
Whichones?

A:
Theoneswe ordered last week. The musicals.

We don’t use one to refer back to uncountable nouns:

A:
Have you got any milk?

B:
Yes.

A:
Can I borrowsome?

Not: Can I borrow one?

As a noun substitute, we don’t use ones immediately after some, any, both and numbers, unless it is premodified:

A:
Are there any mangoes for dessert today?

B:
Yes, Nuala boughtsomeat the supermarket.

Not: … bought some ones at …

A:
How many pens did you buy?

B:
I bought four greenonesand six redones. That should be OK, shouldn’t it? (The underlined words are premodifiers.)

In informal English, my, his, her, etc. (possessive determiners) can come before one. This makes the statement more emphatic:

A:
I couldn’t use my laptop today.

B:
Neither could I. My onegot stolen. (more emphatic than Minegot stolen.)

When one refers back to a previous noun and is premodified, a determiner (a/an, the, this, your) must be used:

The hotel that we had booked turned out to be a luxuryone.

Not: The hotel that we had booked turned out to be luxury one.

I liked all your songs, but Yesterday was your bestone.