情态动词
1. can / could a. 表示能力
He can speak a little Japanese in addition to English.
Man cannot live without air．
Can you finish this work tonight?
b. 表示许可
You can(not) have the book.
Can I use your phone? could也可表示请求，语气更委婉，主要用于疑问句，不可用于肯定句，答语应用can (即：could不能 用于现在时态的简略答语中)，如： --Could l come to see you tomorrow morning7
--Yes，you can.(否定答语可用No，I'm afraid not．) Could you please …?
Could I use your dictionary?
--- Yes, you can. c. 有时候会
He is a bad-tempered fellow, but sometimes he can be charming.
They can be very valuable. d. 在否定句和疑问句中表示猜测(can’t不可能)
Someone is knocking. Who can it be?
It can’t be Mr. Li. Because he is away on business.不可能，肯定不
It can’t have rained last night, for it is dry outside. e. 与possibly 连用; 用于as…as one can
Can/could sb. possibly Can he possibly come? What on earth can she possibly mean?
I will come back as soon as I can.
Sb. can't/couldn't possibly某人不可能
I can't possibly allow you to