Ответ:
Кажется, примерно так.
1.My sister said that the medicine had done a lot of good to her.
2.My friend said that the doctor had checked his lungs the day before.
3.She said that he stayed away from classes for three days.
4. He said his son had a bad cough.
5. John said he had a splitting headache.
6. Mary often told me that she did not like to call for the
doctor.
7.Tony told me that he had all the symptoms of influenza.
8.The doctor said that he had checked her lungs.
9. Mary said that her mother had fallen ill.
10.My brother said that he would follow the doctor's instructions.
11. She said her brother would recover soon.
12. He said he woud take a table spoonful of that mixture
three times a day before meals.
Объяснение:
Времена переходят в соседнее прошедшее, например
Present Simple-Past Simple
Present Perfect-Past Perfect
А предложения в будущем времени с will меняются на would.