1.情态动词有过去式
can - could,may-might,shall-should,will-would
have to - had to
2.情态动词后面跟的行为动词（实意动词）只能用原型,但有时态和语态的变化.
如：He can carry the heavy box.carry 用原型.
He must have returned at that moment.have(原型）returned 完成时
3.晴天动词本身没有人称数的变化.
如：I can carry the heavy box.
He can carry the heavy box.