【问】 ... he often has to look for relationships in data…据我所知，像often这样的频率副词，应当搁在实义动词之前、助动词或联系动词之后。在上面的句子里为什么频率副词搁在助动词have to之前呢?

【答】 由于have to与一般的助动词在语法特点上并不完全相同，所以语法学家们把它划为半助动词。它与副词的相对位置，就是与一般助动词的区别之一。副词(often，frequently，usually，apparently等)应当搁在have to之前。例如：

1. We apparently have to go.

很显然我们必须去。

2. People travelling long distances frequently have to decide whether they would prefer togo by land，sea，or air. (New Concept English，Book 3，L. 44)

人们作长途旅行时，往往不得不决定究竟取道陆路、海洋还是航空。

3. In order to follow the buffalo，the Indians often have to move their camps. (DAI)

为了跟随野牛，印第安人常常不得不迁移营地。