once is one of the 1000 most common headwords.
on - once - one
English numbers (edit)
1 2  → 
    Cardinal: one
    Ordinal: first
    Frequency: once
    Multiplier: single
    Distributive: singly
    Fractional: whole
    Number of musicians: solo

Pronunciation

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Determiner

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Determiner
once

  1. (indefinite) one time
    We go to the house once a year.
    She had only seen him once.
    It's nice to be understood for once.
    We get together once a month for coffee.
    Not once have I said that.
  2. (indefinite) at a time in the past, but not now
    It was hard to believe that once we had been friends.
    The once great city had be destroyed.

Preposition

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Preposition
once

  1. after
    Once I've finished cleaning up, we can leave.
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  NODES
Note 1