1. I am sorry, but I (can’t, must, should) help you. I am very busy now.
2. I (can, can’t, must) find these books. I can’t do this work without them.
3. You (should, must, can) be politer.
4. (must, could, ought to) you lend me your book for some time?
Объяснение:
1.can't
2.must
3.should
4.could