...
For measuring the times of repeated actions
去一次 qù yí cì / qù yi cì
做一次 zuò yí cì / zuò yi cì
试一次 shì yí cì / shì yi cì
...
Used when describing a story which happened in the past
有一次 yǒu yí cì
她从来没被抓过，不过有一次，警报响了。 Tā cónglái méi bèi zhuāguo, búguò yǒu yí cì, jǐngbào xiǎng le.
'She never got caught but, once, the alarm went off.'