情态动词在间接引语中都可以改为过去式，must表示必、推测、禁止等意思时，可不变；表示必须时可不变，也可用had to 或would have to。
the senior said, all men must die.
→ the senior said that all men must die. 那老者说人总是要死的。
she said, i must go to see the doctor tomorrow afternoon.
→ she saidshe mustwould have to go to see the doctor the next afternoon. 她说第二天下午她一定得去看医生。
he said to me, we used to go there every year.
→ he toldme that they used to come here every year. 他对我说他们过去每年都来这里。（used to在间接引语中不变）
i said, you did quite well in the exam yesterday.
→ i said that you had done quite well in the exam the day before.我说你那天考得不错。
he said to tom, i'll do my best to catch up with others.
→ he told tom that he would do his best to catch up with others. 他告诉汤姆他将尽他所能赶上其他人。
he said to her, where did you put the glasses
→ he asked her where she had put the glasses. 他问她，她把杯子放哪儿了。