一旦yídànonce (sth happens, then...); if; in case (sth happens, then...)HSK 5PronunciationyídànExample sentence一旦开始做一件事,就要坚持到底。Yídàn kāishǐ zuò yí jiàn shì, jiù yào jiānchí dàodǐ.Once you start doing something, you must stick with it to the end.Read and choose an answerOne fixed sampleExplore a sample