r. on one occasion
r. as soon as
r. at a previous time;
adv. 一次, 曾经, 一旦
conj. 一旦, 一经
n. 一次
a. 从前的
1. I have only been to Paris once.
我只去过巴黎一次。
2. Once you finish your homework, we can go out.
一旦你完成作业，我们就可以出去了。
3. She called me once last week.
她上周给我打过一次电话。