Words like ‘once’ and ‘twice’ can be predeterminers as multipliers. For example:
“Once every two weeks” “Twice a day”
A search in the iWeb corpus for:
once _AT1 _NNT
1 ONCE A WEEK 75774
We are proposing to meet at least once a week
and just to do as many collections as we can.
2 ONCE A MONTH 44242
3 ONCE A YEAR 43510
4 ONCE A DAY 31663
5 ONCE AN HOUR 1639
I might call you once a week or once an hour.
6 ONCE EVERY WEEK 1285
7 ONCE EVERY YEAR 1201
Once every year,
all the girls from our church get together
8 ONCE EVERY MONTH 1088
9 ONCE A FORTNIGHT 963
10 ONCE EVERY DAY 921
11 ONCE A TIME 744
12 ONCE EVERY HOUR 624
13 ONCE A SEASON 546
14 ONCE A MINUTE 458
15 ONCE A DECADE 454
16 ONCE A WHILE 421
17 ONCE A NIGHT 288
18 ONCE EVERY MINUTE 157
19 ONCE A SECOND 145
However, ‘once’ is very often a subordinating conjunction or general adverb.
1 ONCE (CS@_RR) 2000496 | 3 ONCE (RR_CS@) 994010 = The Claws7 tagger cannot decide if it is a conjunction or adverb.
Here is an advanced example without a verb before the comma. ‘once there’ = ‘once they are there’ = AS SOON AS THEY ARE THERE:
Once there, they interact with receptors on neurons,
changing the way the individual cells work and function
by making them more or less excitable,
altering their growth, or reshaping their connections.
A search in the iWeb corpus for ‘once’ as a subordinating conjunction:
* * once_CS * *
1 . ONCE YOU HAVE 47191
2 . ONCE YOU’VE 18318
3 . ONCE YOU ARE 7623
4 . ONCE YOU GET 5843
5 . ONCE YOU’RE 4398
6 , BUT ONCE YOU GET 3911
7 ? ONCE YOU HAVE 3612
8 . ONCE YOU KNOW 2787
9 . ONCE WE HAVE 2592
10 : ONCE YOU HAVE 2484
We see that punctuation, or evidence that it is at the start of a clause. So we can stretch the search with:
. once_CS * * * *
1 . ONCE YOU DO THAT, 1321
2 . ONCE YOU HAVE DONE THIS 1192
3 . ONCE YOU’VE DONE THAT 1164
Once you‘ve done that, it‘s pretty straightforward from there.
4 . ONCE YOU GET TO THE 1079
5 . ONCE YOU HAVE COMPLETED THE 1034
6 . ONCE YOU GET THE HANG 1019
7 . ONCE YOU GET USED TO 947
8 . ONCE YOU HAVE DONE THAT 734
9 . ONCE YOU DO THIS, 675
10 . ONCE YOU ARE DONE, 620
Now we look at examples where ‘once’ is clearly a part of an adverbial phrase:
2 ONCE (RR) 1897790
in a land very very close to us,
Every once in a while, it would work.
they can be watched from here.
4 ONCE (RR21) 550086
1 ONCE AGAIN 452522
And once again I find myself sitting at the bar.
2 ONCE MORE 86793
I‘ll ask you once more, Aziz, and only once.
5 ONCE (RR22) 257464
1 AT ONCE 222137
They‘re searching the building.
We must leave at once.
2 FOR ONCE 29063
6 ONCE (RR33) 55643
1 ALL AT ONCE 54444 (if the meaning is ‘suddenly’ then this is C1:
They didn’t say I had to do it all at once.
7 ONCE (NN1) 46754 The examples as nouns seem incorrect
8 ONCE (RR41) 31200
1 ONCE AND FOR ALL 30247
Now, I have here a letter that will prove,