核心问题
People travelling long distances frequently have to decide whether they would prefer to go by land, sea, or air. (New Concept English, Book 3, L.44)
句中的副词 frequently 是修饰哪个成分的？是修饰 travelling，还是修饰 have to?
词典解答
从句意上看，把 frequently 看作是 travelling 的修饰语，还是 have to 的修饰语都是讲得通的。但是，从语法上看，frequently 只能是修饰 have to 的。根据英语惯用法，have to 的状语要放在它的前面。例如：
We apparently have to go. (AEG)
但是,在其他情况下(即 have 不带 to 时),状语要放在 have 的后面:
-
He has apparently gone. (AEG)
-
I have never mentioned it.