can/could
1) 表能力=be able to
Can you dance? Yes, I can. / No, I cant.
2) 表请求，允许 (could语气较委婉）
Can I use your pen? Yes, please./No, Im afraid not.
3）表猜测，可能，多用于否定句及疑问句，并表不太可能。（could可表虚拟）
He cant be at home now.
What can he be doing?
4）其它常用语
I cant help laughing. (禁不住）
I cant but ask him about it.（只得）
may/might
1) 表猜测
He may not know it.
She may be ill.
2) 表请求，许可
May I use your pen?
May I smoke here? Yes, you may. / No, you mustnt (cant)
*3) 表责备，只用might，且用于肯定句。
You might ask before you borrow my car.
You might have helped me.
*4) 其它用法
You may as well give him the letter. (还是）
His appearance has changed so much that you may well not recognize him.(很可能）
must
1）表命令，许可 = have to
Must I go now? Yes, you must. / No, you neednt.
mustnt意为不许
You mustnt smoke here.
2) 表猜测，可能性较大
He must be an actor.
need
表必须，仅用于否定句或疑问句中，在肯定句中一般用must, have to, ought to,或should代替。
You neednt come so early.
Need I finish homework today? Yes, you must.
shall/should
1）表命令，在某种程度上相当于must，ought to
You should do as I told you.
2) 表建议，要求
You should tell the the police.
3) 询问意见
Shall I get you some fresh coffee?
will/would
1）表意愿
I wont argue with you.
2) 表请求，邀请
Will you go with me?
Would you please close the door?
今天做得头发大了，其余的以后再慢慢加上吧！