时态的比较：
时态的比较一般指的是动作或状态发生的先后。
过去时比现在时先发生：
例句：1. Jack built a path last year, and now he builds a house. [动作]
2. Mr. White was a worker years ago, and now he is an engineer. [状态]
完成时要比一般时先发生：
例句：Tom said that he had done his work already.
现在完成时与过去时不能比较，因为完成时指的是范围，而过去时指的是时点。
将来时比一般时晚发生：
例句：The mother told us she would prepare the dinner early tomorrow.
时态的统一：
同一句子里的两个谓语要保持逻辑上的统一。但当提到客观真理时，可以不管这些，直接用一般现在时。
例句：1. Mary said that she wanted to have a look at the picture.
2. They found that the house would be too small soon.
3. The professor told me that the earth is a planet of the sun.