英语中的情态动词有多种过去式形式，如can变为could，may变为might，shall变为should，will变为would，must变为had to，ought to变为ought to have。掌握这些过去式可以帮助我们更准确流畅地表达过去的事情或假设。
- can → could
- may → might
- shall → should
- will → would
- must → had to
- ought to → ought to have
其中，can的过去式could可以表示“过去能够做某事”，例如：I could swim when I was young. may的过去式might可以表示“过去可能做某事”，例如：He might have gone to the party last night. shall的过去式should可以表示“过去应该做某事”，例如：I should have studied harder for the exam.
将来式的will的过去式would可以表示“过去愿意做某事”，例如：He would always help me when I needed it. 在过去式中，must的过去式为had to，表示“过去必须做某事”，例如：I had to work extra hours last week. ought to的过去式ought to have可以表示“过去本应该做某事”，例如：I ought to have called her yesterday.
