Use 'once' to talk about something that happened a single time in the past. It means one time only.
I saw that movie once last year.
She called me once yesterday evening.
You use 'once' to mean that when one thing happens, another thing happens right after. It shows order in time.
Once you finish, please close the door.
Once it rains, the grass grows fast.
'Once' here talks about something true in the past but not now. It tells about a previous time or condition.
He was once a famous singer.
This building was once a school.
'Once' in this sense means sometimes or occasionally but not often. It shows something happens now and then.
Once in a while, I like to eat ice cream.
We go hiking once in a while.
This idiom uses 'once' to mean doing something one time in a final way. It shows no more actions will follow.
Let's settle this once and for all.
He cleared his debts once and for all.
'Once' can be a noun meaning a single event or occasion, mostly in old or literary English. It is rare in modern speech.
He was there but only for a once.
Such once are rare in history.
