Question

关于英语语法的问题，急~

I am used to travelling by air and only on one occasion have I ever felt frightened.
后半句的语法，倒装句那个，为什么？
谢谢！

Answer 1

only在句中强调状语放在句首后面的句子要倒装，状语可以是介词短语，也可以是从句，如：
Only in a big city was it possible to buy a new wheelchair.
只有在大城市里才能买到新轮椅。
only强调状语in a big city,后面的句子用倒装，was it possible to buy a new wheelchair.

Only when we had studied the data again did we realize that there was a mistake. 只有当我们再次研究了这些数据的时候， 我们才意识到出了一个错。
only强调状语when we had studied the data again，后面的句子要倒装：did we realize that there was a mistake.

Answer 2

only 放在从句聚首后面要倒装
意思是我经常乘飞机出行，仅有一次感到恐怖

Answer 3

上面第三位的回答很好。
补充一点：
当Only修饰副词，放于句首时，主句也需要部分倒装的。
如：Only then did I realize that I had made a mistake.

Answer 4

only + 状语 之后要 部分倒装

on one occasion 就是状语