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.