Then he spoke slowly, but I could not understand him

问题描述:

Then he spoke slowly, but I could not understand him
Then he said something and I understood it
这同一篇文章的两句话.为什么第一句是understand , 第二句是understood
整篇都是过去时,怎么第一句使用understand.

很简单,因为第一句understand前面是情态动词could,而情态动词后面必须跟动词原形.