The boy must have chosen this book by himself. If he had been helped by adults, they would have never advised him to take such a boring book.
3. Your friend should have gone to a doctor long ago. If he had been treated, he would have recovered from the illness long ago.
4. He, perhaps, had tried to prove to her that he was right, but he could not. She is very difficult to convince of anything.
5. He is unlikely to argue about this proposal. If he had any objections, he would have tried to find out everything during the last discussion.
6. A delegation may arrive at any time. You should prepare all the necessary documents very quickly. If I had known about this before, I would surely have warned you beforehand.