直接引语变成间接引语时，从句时态无须改变的情况 1）当主句的谓语动词是一般现在时的时候，如： He always says, “I am tired out.” ——>He always says that he is tired out. 2）当主句的谓语动词是将来时的时候，如： He will say, “I'll try my best to help you.” ——>He will say that he will try his best to help me. 3）当直接引语部分带有具体的过去时间状语时，如： He said, “I went to college in 1994.” ——>He told us that he went to college in 1994. 4）当直接引语中有以when, while引导的从句，表示过去的时间时，如： He said，“When I was a child, I usually played football after school.” ——> He said that when he was a child, he usually played football after school. 5）当直接引语是客观真理或自然现象时，如： Our teacher said to us, “Light travels faster than sound.” ——> Our teacher told us that light travels faster than sound. 6）当引语是谚语、格言时，如： He said，“Practice makes perfect.” ——>He said that practice makes perfect. 7）当直接引语中有情态动词should, would, could, had better, would rather, might, must, ought to, used t-o, need时，如： 例如： The doctor said, “You'd better drink plenty of water.” ——> The doctor said I'd better drink plenty of water. He said, “She must be a teacher.”——> He said that she must be a teacher. He said, “She ought to have arrived her office by now.”——> He said that she ought to have arrived her office by then. The teacher said, “You needn't hand in your compositions today.”——> The teacher said we needn't/didn't need to/didn't have to hand in our compositions. She asked, “Must I take the medicine?”——> She asked if she had to take the medicine. 〔注〕：此处用had to代替must更好 8）此外转述中的变化要因实际情况而定，不能机械照搬，如果当地转述，here不必改为there, 动词come不必改为go，如果当天转述yesterday, tomorrow, this afternoon等均不必改变。如： Teacher: You may have the ball game this afternoon. Student : What did the teacher say, Monitor? Monitor: He said we might have the ball game this afternoon.
除了真理，还有很多情况是不变换时态的，要看情况讨论，不知道对你有没有帮助呢？
参考资料：资料来自百度百科