(1) adv. 一次
I've only been to Brighton once.
I have seen her once only.
I've been there more than once.
(2) adv. 从前；曾经
at some time in the past
He was once a taxi driver;now he is a college student.
There once lived a man who had two beautiful daughters.
I was very fond of him once.
once every other day
once a day
Not once have you done as I have asked.
(3) conj. 一旦；一…就
as soon as
Once water begins to boil,its temperature no longer rises.
Once seen,it can never be forgotten.
Once you show any fear,he will attack you.
(4) n. 一次
one time,on one occasion
Perhaps just this once,we'll use what we have to get a new dress for you.
Once is enough for me.
She did it just once.