once的用法及时态总结如下：
Once you start, you will never give up.（一旦开始，就不要放弃。）
Once you begin to read it, you will like it.（一旦开始读，就会喜欢它。）
Physics is easy to learn once you understand the rules.（一旦理解规则，物理就不难学。）
Once he lived in America, but now he lives in England.（他曾经住在美国，现在住在英国。）
His song was once very popular among the young people.（他的歌一度在年轻人中很流行。）
My father once employed a young man named Johnson.（我父亲曾雇佣过一个叫约翰逊的小伙子。）
The old professor comes to see us once a week.（老教授每周来看我们一次。）
Tell us the story once more.（把故事再讲一次。）
例：I once went to Shanghai.（我曾经去过上海。）
例：I went to Shanghai once.（我去过上海一次。）
All of you should go there at once.（你们所有人应立刻去那里。）
Finish the task at once.（请立刻完成任务。）
Let me hear you sing once again/once more.（让我再听你唱一遍。）
Read the passage once more.（把课文再读一遍。）
We went to see our English teacher once in a while.（我们偶尔去看英语老师。）
All at once, they rushed out altogether.（突然，他们一起冲了出去。）
Once upon a time there was an old man named John Hill.（从前有个叫约翰·希尔的老人。）
