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.
'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.
