在 'We ought to pay' 中，'ought' 是一个情态动词。

In the sentence 'We ought to pay', 'ought' is a ___modal.

'Must' 是英语中常见的情态动词。

'Must' is a common ___modal in English.

学习英语时，理解如何使用情态动词很重要。

When learning English, it's important to understand how to use a ___modal.