Once when we were going to Paris, I forgot my passport. 有一次我们准备去巴黎，我忘了带护照。

【参考解析】这句话是正确的。也可以写作：

Once, when we were going to Paris, I forgot my passport. 有一次我们准备去巴黎，我忘了带护照。即，实际上这个句子带有两个时间状语，一个为 once（有一次）；另一个为when 从句。前者表达的时间较为模糊，有时有承接上文举例之用；后者则更加明确事件发生的时间。

 【更多例句】

1. Once when I was a teenager, my father and I were standing in line to buy tickets for the circus. Finally, there was only one family between us and the ticket counter. 记得我少年时有一次和父亲排队买看马戏的票，终于我们前面只剩下一家人了。

2. Once when I was flying home the pilot had to crab the plane.曾经有一次，当我乘飞机回家的时候，飞行员不得不让飞机偏航。

3. Once, when I was little, I used to feel so strong;

    Yea once, when I, I was little, I could dream more then.

4. Once when Jacob was cooking some stew, Esau came in from the open country, famished. 有一/次天，雅各熬汤，以扫从田野 回来累昏了。