Once

We useonceas anadverb to mean ‘one single time’:

I’ve only met Jane’s husbandonce.(one time)

We sayonce a+ singular time expression andonce every+ plural time expression to talk about how often something happens:

They go for dinner togetheronceamonth.(one time per month)

Not:They go for dinner once the month.

I see himonce everytwo or three weeks.

We also useonceto mean ‘at a time in the past but not now’. In this meaning, we often use it in mid position (between the subject and the main verb, or after the modal verb or first auxiliary verb, or afterbeas a main verb):

My fatheronceworked on an oilrig.(He no longer works there.)

The Millersonceowned a dairy farm.(They no longer own a dairy farm.)

She wasoncea schoolteacher but she hated it.

The phraseonce upon a timeis used at the start of children’s stories. We sometimes use it to mean ‘long ago’:

Once upon a timethere was a little girl called Little Red Riding Hood …

You used to go to nightclubsonce upon a time!

We useonceas a conjunction meaning ‘as soon as’ or ‘after’:

OnceI’ve picked Megan up, I’ll call you.

My boss is a nice manonceyou get to know him.

We don’t useshallorwillin the clause withonce:

OnceI pass all my exams, I’ll be fully qualified.

Not:Once I will pass…