大学英语六级基础语法知识：时态

　　--He said he had been collecting materials on that subject for a long time.(过去进行时变为过去完成进行时)

　　--He told me that they would have completed that project by the end of the year.(将来完成时变为过去将来完成时)

　　虽然主句用了过去时等，如果从句表示的是科学真理、客观事实或某人(物)的经常性特点时，其时态可以不必遵守时态呼应的规则而仍用一般现在时。

　　--Galileo proved that the earth revolves round the sun.

　　--The teacher told the students that knowledge is power.

　　如果从句中有一个表示绝对过去时间的状语，就不必把一般过去时变为过去完成时。

　　--He said that he joined the Red Army in 1933.