一旦

一旦	yī dàn	in case (sth happens) / if / once (sth happens, then...) / when / in a short time / in one day
一	yī	one / single / a (article) / as soon as / entire; whole; all; throughout / "one" radical in Chinese characters (Kangxi radical 1) / also pr. [yao1] for greater clarity when spelling out numbers digit by digit
旦	dàn	(literary) dawn; daybreak / dan, female role in Chinese opera (traditionally played by specialized male actors)