一般分三种情况
主句中的谓语是一般现在时,引语部分的时态不变 e.g: She says ,"she is happy."----> She says she is happy.
主句中的谓语是一般过去时,引语部分的时态是一般过去时 e.g:She said ,"she is happy." ---->She said she was happy
引语如果陈述的客观事实或真理,永远用一般现在时. e.g:The teacher said," the moon goes around the earth." ---->The teacher said the moon goes around the earth.