modal       
noun [ C ]  language specialized uk/ˈməʊ.dəl/ us/ˈmoʊ.dəl/ (also modal verb)
B1 情态动词
a verb, such as "can", "might", and "must", that is used with another verb to express an idea such as possibility that is not expressed by the main verb of a sentence
  The first verb in the following sentence is a modal: We ought to pay the gas bill. 下面句子中的第一个动词是情态动词:We ought to pay the gas bill. 
 - More examples
 - We did some exercises on modal verbs.
 - Modal verbs can be quite tricky.