It is her eraser 怎么改成否定句

It is her eraser 怎么改成否定句 it is her eraser改为否定句 this is my eraser.改成否定句 That is her eraser改为否定句 It is my eraser.改否定句 It is an eraser .改为否定句 Is your last name Green?(改为否定句） It is her red eraser(改为同义句） This is my eraser 改成否定句 ——my eraser . It is her red eraser.(改为同义句) That is her backpack 改成否定句 Tom has an eraser.改成否定句 It's her eraser 改为否定句 It‘s isn’t eraser 为什么用isn‘t 而不用not