Once

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Once has two uses:

1. as an adverb of frequency  ( = on one occasion )
2. as an adverb of time ( = at some time in the past )

1. Once as an adverb of frequency

Once when acting as an adverb of frequency usually goes at the end position.
Example:
Q: How many time have you visited Thailand?
A: I've visited there (only) once.

There're other expressions of frequency containing once:
Once a day, once a week, once a month, once a year

Note: In counting the number of times or occassions, we say: once, twice, three times, four times, five times, ... . We almost never say: one time, two times, ...

2. Once as an adverb of time

Once as an adverb of time is usually in front or middle position. Its' meaning is "at some time in the past". It goes before a past tense verb.
Example:
Once I stayed in an apartment.
She once drew a nice picture.

3. Idioms

At once, for once, once again, once more, once or twice, once upon a time ...

These idioms can be found in dictionaries or online sources.

Once Upon A Time - Max Oazo ft. Moonessa (Lyrics + Vietsub) ♫