间接引语中的must
 【问】 有的教科书上说，直接引语中的must在间接引语中必须换成had to。果真如此吗?
 【答】 在间接引语中，must既可以用had to来代替，也可以保持原形不变。
 1. His parents told him that he be in by ten that night. (CGEL)
 他的父母告诉他那天晚上他必须10点以前到家。
 2. “I must go. ”→
 He said he must[had to]go. (PEU)
 “我必须去。”
 他说他必须去。
 3. He said he
 他说他必须早走。
 4. She said they must go early the next day. (LEG)
 她说他们第二天必须早走。
 5. He said that they must be hungry.
 他说他们一定饿了。
 据L. G. Alexander的解释，在间接引语中使用must与使用had to在意义上是有区别
 的：
 He said he must leave early.(说此话时他尚未离开)
 他说他必须早走。
 He said he had to leave early.(说此话时他已离开)
 他说他不得不早走。
 此外，need，ought to，had better本身没有过去时形式，因而它们在间接引语中仍保留其原有形式。
 1. He warned me that I had better not say anything about that. (CGEL)
 他警告我最好不要对那件事说什么。
 2. The doctor told him (that) he ought to slow down a bit. (LEG)
 医生告诉他应该放慢一些节奏。
 3. The boss said I needn't come in the next day. (LEG)
 老板说我第二天不必来了。