几道关于情态动词的英语题目：
1.Would you be so kind as to keep an eye on my house while I am away?
(so+adj.+as to do sth怎么理解,这里的Would 能不能改为Can或Could,原因）
2.Should the pain return,take one of these pills.怎么理解这个should
3.He ordered that Tom should leave th house at once.这里的should可以换成must吗?原因.
4.There is a knock at the door.Who can it be?这里的can能改成may?原因.
5.-Can you go swimming wiht us this afternoon?
-Sorry,I can't.I ____ take care of my little sister at home because my mother is ill.
A.can B.may C.would D.have to
6.-Mr.Smith must have been to your home this morning.
-No,he ______,because he didn't know my address.
A.couldn't B.can't C.mustn't D.may not