直接引语和间接引语。

1. 相关语法规定：如果主句的时态为一般过去时，而直接引语变成间接引语时，引语的时态要在原来的时态基础上再加上过去的概念。

2. 所以，如果直接引语本身就是一般过去时的话，则变成间接引语时则其时态变成“过去的过去”，就是所谓的过去完成时了。

3. 而过去完成时则要求句子的谓语构成为：had + 动词的过去分词。

直接引语变为间接引语时,通常受转述动词 said,asked 等的影响而使用过去化的时态, 即把原来的时态向过去推,也就是一般现在时变 一般过去时,现在进行时变为过去进行时等. 例如: Tom said to me,"My brother is doing his homework." →Tom said to me that his brother was doing his homework

直接引语在改为间接引语时、时态需要做相应的调整。 现在时它需改为过去时态；过去时态改为完成时；过去完成时则保留原来的时态。如： 1)She said. "I have lost a pen."→She said she had lost a pen 2)She said. "We hope so."→She said they hoped so. 3) She said. "He will go to see his friend。"→She said he would go to see his friend。

简单的说就是把时间点往前推一格，当然也有例外比如客观真理就不用变时态

过去的过去用过去完成时态，had+done