Ответ:

a)

1. My sister can type very well now.

2. We was not able to hear a word.

3. I think, I'll be able to see you only tomorrow.

4. Some years ago I could speak English very well, but now I'm completely out of practice.

5. Do you think that he can finish this work next week?

b)

1. You may take my pencil, if you haven't got one.

2. You may go.

3. Yesterday we was allowed to go home at 3 o'clock.

4. He had an infectious disease and was not allowed to go out.

c)

1. You must be in time for the lesson.

2. The weather was bad and we had to stay at home.

3. They must go there at once.

4. He must explain everything to us when he returns!

5. The train must leave at 8:30.

6. He was to come at 10 in the morning yesterday, but he didn't come at all.