1.That is an eraser.改为否定句That is not an eraser.2. 英语中的肯定句改成否定句的方法：在be动词, 助动词，情态动词后面加not。

这个句子中含有be动词is，所以在它后面直接加not就可以了。

3. 其它两种类型的否定句例子: Lily goes to school by bike every day.改为 Lily doesn't go to school by bike every day.They can swim.改为They can't swim.