Давайте разберем, как преобразовать прямую речь в косвенную. Для этого нам нужно учитывать несколько важных моментов:

• Изменение местоимений: Заменяем местоимения в зависимости от того, кто говорит и к кому обращаются.
• Изменение времени: В большинстве случаев, время в предложении изменяется на одно время назад.
• Структура вопроса: В косвенной речи обычно используется структура утвердительного предложения, а не вопросительная.

Теперь давайте преобразуем каждое предложение:

1. I said to Nick, "Where are you going?" → I said to Nick where he was going.
2. I said to him, "How long are you going to stay there?" → I said to him how long he was going to stay there.
3. I said to him, "How long will it take you to get there?" → I said to him how long it would take him to get there.
4. He said to her, "Where do you usually spend your summer holidays?" → He said to her where she usually spent her summer holidays.
5. Ann said to Mike, "When did you leave London?" → Ann said to Mike when he had left London.
6. She said to Boris, "When will you be back home?" → She said to Boris when he would be back home.
7. Boris said to them, "How can I get to the railway station?" → Boris said to them how he could get to the railway station.
8. Mary asked Tom, "What time will you come here tomorrow?" → Mary asked Tom what time he would come there the next day.
9. She asked me, "Why didn't you come here yesterday?" → She asked me why I hadn't come there the day before.
10. She asked me, "What will you do tomorrow if you are not busy at your office?" → She asked me what I would do the next day if I was not busy at my office.
11. Pete said to his friends, "When are you leaving St. Petersburg?" → Pete said to his friends when they were leaving St. Petersburg.
12. He said to them, "Who will you see before you leave here?" → He said to them who they would see before they left there.
13. They said to him, "What time does the train start?" → They said to him what time the train started.
14. I asked Mike, "What will you do after dinner?" → I asked Mike what he would do after dinner.
15. I asked my uncle, "How long did you stay in the Crimea?" → I asked my uncle how long he had stayed in the Crimea.
16. Mother said to me, "Who has brought this parcel?" → Mother said to me who had brought that parcel.
17. Ada said to me, "Where did you see such trees?" → Ada said to me where I had seen such trees.
18. I said to Becky, "What kind of book has your friend brought you?" → I said to Becky what kind of book her friend had brought her.

Таким образом, мы преобразовали все предложения из прямой речи в косвенную, следуя указанным шагам. Если у вас есть вопросы по какому-либо из предложений, не стесняйтесь спрашивать!