once, one time (one and only one time) / once (formerly; during some period in the past) / sometime (at an indefinite but stated time in the past or future) / first of all, for starters, firstly (as an initial point; before any other considerations)

one more time, again

not even

at once, at a time, in one go / all at once, all of a sudden, suddenly

Expresses surprisal and scepticism at a new development, especially someone’s changing their opinion

just, simply

Once doesn’t count; one swallow does not a summer make / One slip is allowed; everyone deserves a second chance / One time won’t hurt; just try it

(used at the beginnings of fairytales) once upon a time

non-recurring, one-time / singular, unique / unheard-of

(arithmetic) multiplication table; times table / (figuratively) basic knowledge; 101