副词 adv.( 词义：一次,一回):
I see them once every two months.
我每两个月与他们见一次面.
连词 conj.（ 词义：一旦,一经...便）
Once you get into a bad habit, you'll find it hard to get out of it.
一旦染上坏习惯,想改掉就难了.