once

词义辨析：

同义:

ad.从前地

previously

aforetime

heretofore

in the past

at one time

long ago

ad.一度, 一次

single time

one time

on one occasion

one time only

never again

ad.曾经, 一度

ever

at any time

at a single time

by chance

by a fluke

n.一次

single

occasion

single time

one time only

同义参见:

formerly

ago

反义参见:

again