有读者问这样一句话：
(1)The people on the train told the boy not to shout, but he insisted that there was danger ahead and that the train must stop at once.
我们认为句中的must应改为had to。不知正确否？
这个句子中的insisted意谓“坚持说”(declared firmly)，从句that the train must stop at once是它的第二个宾语。这个宾语从句和前一个宾语从句一样，都是间接引语。我们知道，当must意谓“必须”时，在间接引语中它的过去式仍是must，这就是你所问的为什么你所引的句子用must的缘故。这样的must也可改为had to。再如：
(2)He said he must go tomorrow.
(3)I decided that I must stop smoking.
