求英语高手识别一下这三道题共同属于哪一类语法点(语法知识)?

题目

求英语高手识别一下这三道题共同属于哪一类语法点(语法知识)?
19.Read it aloud so that I ____ you clearly.
A.may hear B.will hear C.hear D.have heard
20.They got up early in order that they ____ they first train.
A.caught B.will catch C.might catch D.shall catch
21.I am sorry that he ____ in such poor health.
A.are B.shall be C.were D.should be

答案

情态动词