Meaning of yī dàn:
yī dàn
一旦
Adverb
in case (sth happens); once (sth happens, then...); when; in a short time; in one day
Yī dàn xià yǔ, jìdé dài sǎn.
一旦下雨，记得带伞。
In case it rains, remember to bring an umbrella.
Yī dàn xià yǔ, xíngrén jiù huì chēng kāi yǔsǎn.
一旦下雨，行人就会撑开雨伞。
Once it starts raining, pedestrians will open their umbrellas.
Yī dàn nǐ kāishǐ, nǐ jiù bìxū wánchéng tā.
一旦你开始，你就必须完成它。
Once you start, you must finish it.
Yī dàn xià yǔ, wǒ men jiù bù néng qù gōng yuán.
一旦下雨，我们就不能去公园。
As soon as it rains, we cannot go to the park.
yi1 dan4 ni3 jue2 ding4 le, jiu4 bu2 yao4 hou4 hui3.
一旦你决定了，就不要后悔。
Once you have decided, do not regret it.
Conjunction
if
Yī dàn xià yǔ, wǒmen jiù bùnéng chūqù wán.
一旦下雨，我们就不能出去玩。
If it rains, we cannot go out to play.
