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 Come up and see me once.)
  We must go walking one day. (not We must go walking once.)

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.’