情态动词这过去式不等于过去的时间，它代表着常常是距离distance。
can的过去式是could，will的过去式是would,may的过去式是might。。例句如下:
Could you give me a pen?
Would you open the window?
I wanted to ask you something.I was wondering if you could help me.（这里的过去式都表示委婉,用过去式拉远距离,更加礼貌。。）
"You may go to the party tonight, but you might feel tired tomorrow if you stay up too late."在这个句子里，May = 可能性较大，或表示允许。Might = 可能性较小，语气更不确定。
If i were you, i would do.I wish I knew.
这两个都是虚拟语气。
此外:“Might” 除了表示可能性小，在英式英语中也常作为 “may” 的过去式用于间接引语，例如：He said he might come. （他说他可能会来。）
所以我们可以理解按可能性从大到小排列如下：must > will > would > should/ought to > can > may > could > might
