英语语法问题-直接引语-间接引语时为什么时候不变时态?

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.