(1)能，会(indicating physical or mental ability)(can的过去式)：He～play the piano when he was only a child.他小时候就会弹钢琴。She said she～swim very well.她说她游泳很好。(见【惯用法】1))

(2)可以(indicating permission)(can的过去式)：C～I borrow your dictionary，please? 可以借用一下你的词典吗? Mother said I～go to the pictures.妈妈说我可以去看电影。(见【惯用法】2))

(3)请(indicating requests)：C～you help me with this box? 请帮我拿一下这个盒子好吗? If you～just open the door.请开一下门好吗?(见【惯用法】3))

(4)可能(indicating possibility)：You～be right，but I don't think you are.你也许是对的，但我不这么认为。He～n't be her father;he's too young.他不可能是她的父亲;他太年轻了。What～have become of him now? 他现在会是什么样子呢? She～have gone with some friends.她有可能和几个朋友走了。He～n't have heard them because of the noise.那么乱，他不可能听见他们的话。You～have caught the train if you had taken a taxi.你要是乘出租车的话就赶上火车了。You～at least have sent a card.你本来至少可以寄一张贺卡的。

(5)可以，应当(used to make suggestions)：We～go to the cinema tonight.我们今晚去看电影吧!C～we meet again tomorrow? 我们明天再见面好吗? You～write a letter to the headmaster.你可以给校长写封信。

(6)(以便)能够(might;would be able to)(用于目的状语从句中)：He left early so that he～catch his train.他走得很早，以便能赶上火车。

【惯用法】1)could不可表示过去具体某一次行为的能力，此时要用be able to，manage to，succeed in等，如：I was able/managed to finish the homework in an hour.(我能够/设法一小时内把作业做完。)2)could只可用于请求别人“允许”或转述他人“允许”，不可用于表示说话者对他人的允许(见can【辨异】1))。此外，表示具体某一次的允许时，须用be allowed to代替could，如：The children were allowed to(*could) stay up late last night.(孩子们昨天夜里获准晚睡了。)3)表示请对方做某事时，could比can 客气，说话带试探性口吻。

【辨异】1)could与can¹和may (might¹)表示“可能性”时的区别见CAN¹;2)could与can¹和may (might¹)表示“也许”时的区别见CAN¹;3)could与can¹和shall表示建议时的区别见CAN¹。