托福(toefl)語法技巧：句子的時態比較和統一

時態的比較：
　　時態的比較一般指的是動作或狀態發生的先後。
　　過去時比現在時先發生：
　　例句：1. Jack built a path last year, and now he builds a house. [動作]
　　2. Mr. White was a worker years ago, and now he is an engineer. [狀態]
　　完成時要比一般時先發生：
　　例句：Tom said that he had done his work already.
　　現在完成時與過去時不能比較，因爲完成時指的是範圍，而過去時指的是時點。
　　將來時比一般時晚發生：
　　例句：The mother told us she would prepare the dinner early tomorrow.
　　時態的統一：
　　同一句子裡的兩個謂語要保持邏輯上的統一。但儅提到客觀真理時，可以不琯這些，直接用一般現在時。
　　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.