may I do...?
Yes, you may.
No, you can't/ mustn't.
Can I do sth?
Yes, you can.
No, you can't.
Must I do sth?
Yes ,you must.
No, you needn't./ No, you don't have to .
Need I do sth?
Yes , you must.
No, you needn't.
也可以看看这个
—Must
I do it at once?
—No,you
____．
A．mustn't
B．needn't
C．can't
D．don't
此题应选B.一般说来,以must引出的一般疑问句,其否定回答通常是用needn't（=don't
have to）,这与其说是一个语法问题,不如说是一个语境或上下文是否通顺的问题.
注意以下各组问答句,并注意体会其汉语译文：
1．—Must
I come before dawn?我必须天亮前来吗?
—Yes,you
must．是的,必须天亮前来.
—No,you
needn't．不必天亮前就来.
—No,you
don't have to．不必天亮前就来.
2．—May
I smoke in the room?我可以在这个房间抽烟吗?
—Yes,you
may．当然可以.
—Of
course you may[can]．当然可以.
—No,you
may not[mustn't,can't]．不行.（语气较生硬）
—You'd
better not[Better not]．最好不要抽.
—I'd
rather you didn't．还是不抽的好.
—Please
don't．请不要抽.
3．—Need
I finish it today?我必须今天完成它吗?
—Yes,you
must[have to,should]．是的,今天必须完成它.（肯定回答不能用Yes,you
need,因为情态动词need只用于否定句或疑问句,不用于肯定句）
—No,you
needn't．不必今天就完成它.
—No,you
don't have to．不必今天就完成它
