1.can
1）表示能力.“能,会”
Two eyes can see more than one.两只眼比一只眼看得清.
2）表示允许.“可以”
Can I have a look at your new pen?
我可以看一看你的新钢笔吗?
3）表示可能（理论上或是逻辑判断上）.
A 用于肯定句“有时会,一时会”
The monkeys can be very naughty.
The temperature can fall to –60℃,that is 60℃ below freezing.
气温可降至—60℃,也就是零下60℃.
B 用于否定句,疑问句
He can’t have enough money for a new car.
他不可能有足够的钱买新车.
Who can it be?
2.could
1)表示能力,过去时间
Could the girl read before she went to school?这女孩上学前能识字吗?
2）表示允许.
过去时间He asked whether he could take the book out of the reading—room.
现在时间的更委婉更客气的语气：Could I have one?