could 为 can 的过去式.
can 和could 只能用于现在式和过去式两种时态,将来时态用 be able to 来表示.
He could help us at all.
他完全可以帮助我们.
With the teacher's help,I shall be able to speak English correctly.
由于老师的帮助,我将能准确地讲英语.
may (might) 可以,表示说话人同意,许可或请求对方许可.
You may take the book home.
你可以把书带回家去.
You may put on more clothes.
你可以多穿点衣服.
He said he might lend us some money.
他说他可以借给我们一些钱.
may 否定式为 may not,缩写形式是 mayn't.
might 是may 的过去式,有两种用法,一种表示过去式,一种表示虚拟语气,使语气更加委婉,客气或对可能性的怀疑.
He told me he might be here on time.
他说他能按时间来.
Might I borrow some money now.
他可能还活着.