Page 401 - Practical English Usage 3ed - Michael Swan, Oxford

is especially valued - for example the language of religious services, public ceremonies and the law. Some dialects, too, preserve forms which have disappeared from the rest of the language - second-person singular pronouns (tha, thee etc) are still used by many people in Yorkshire.
393 once: adverb
When once means 'at some time', we use it for the past but not for the future. To refer to an indefinite future time, we can use sometime or one day.
Compare:
- I met her once in Venezuela.
Once upon a time there was a beautiful princess.
- Come up and see me sometime. (NOT CBlne r:tp a/'I(1 see me Bnee.) We must go walking one day. (NOT We /mlst gB walking B/zee.)
When once has the more precise meaning of 'one time (not twice or three times)', it can be used to talk about any time, including the future.
I'm only going to say this once.
Note that at once means 'immediately'.
Can I have the bill? - At once, sir. 394 once: conjunction
Once can be used as a conjunction, meaning 'after', 'as soon as'. It often suggests that something is finished or completed, and is most often used with a perfect tense.
Once you've passed your test I'll let you drive my car.
Once he had found somewhere to live he started looking for work. Once you know how to ride a bike you never forget it.
Note that we do not use that after once (NOT Onee that YBlt'lJe passed YBltF fest-. . . ).
For present perfect instead of future perfect after conjunctions. see 580.
395 one: substitute word 1 use
We often use one instead of repeating a singular countable noun. Which is your boy? '" The one in the blue coat.
I'd like a cake. A big one with lots o f cream.
can you lend me a pen? -Sorry, I haven't got one.
2 a ... one
We drop a if there is no adjective. Compare:
I'm looking for a flat. I'd like a small one with a garden. I'd like one with a garden. (NOT •.• a Bne with (j! gdFtien.)
one: substitute word 395
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