We visited the old man yesterday.改为否定句：We didn't visit the old man yesterday.We visited the old man yesterday.改为否定句：We didn't visit the old man yesterday.其中我听别人说时态不变原则,那原句是过去式,改后的句子怎么把visited后面的ed去掉了呢?这是为什么呢,

We visited the old man yesterday.改为否定句：We didn't visit the old man yesterday.
We visited the old man yesterday.改为否定句：We didn't visit the old man yesterday.其中我听别人说时态不变原则,那原句是过去式,改后的句子怎么把visited后面的ed去掉了呢?这是为什么呢,