1、Once 可以用在句首作为连词，引入一个条件或过去式。
2、Once you have finished your homework, you can watch TV. (introducing a condition)
一旦你完成了你的家庭作业，你就可以看电视。（引入条件）
Once upon a time, there was a beautiful princess who lived in a castle. (introducing a time in the past)
从前有一个美丽的公主，住在一座城堡里。（过去式）
Once the contract is signed, we can proceed with the project.
一旦合同签署，我们就可以开始这个项目。
Once the sun has set, the stars will come out.
一旦太阳落山，星星就会出现。
Once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return. (Leonardo da Vinci)
一旦你尝到了飞行的滋味，你就会永远用你的眼睛朝天空凝视，因为你曾经去过那里，而且你总是渴望回去。（达·芬奇）
