Once means ‘on one occasion’.
- I only went once.
Once can also mean ‘at some time in the past’.
- Once (upon a time) there was a princess.
Note that to refer to an indefinite time in the future, we use sometime or one day, not once.
- Come up and see me sometime. (NOT … see me once.)
As a conjunction
Once can be used as a conjunction meaning ‘after’, ‘as soon as’. It is most often used with a perfect tense.
- I would like to go for a walk once the rain has stopped.
- Once he had found somewhere to live he started looking for work.