That is my knife.改复数句怎么做?

That is my knife.改复数句怎么做? this is my knife改复数句 that is my photo改复数 There is a knife on the desk.改复数句 This knife is his.怎么改复数形式的句子? that is a biack knife的复数形式是什么 That is her knife.（改为复数） ____are___ ___ ____ . My knife is over there.(改为复数句） This is a photo of my family (改为复数形式)为什么不改my family为our families?而This isn't my knife改为复数变为our knives? He has a knife.改复数句 这些英语句子怎么改形式?this is a piano（改复数）that knife is short（改复数）is it a spoon?（改复数）are they old cooks?（改单数）are those girls new?（肯定句）are these pencil-boxes old?（肯定句） that is a cup i am a teacher she is my friend 都改复数怎么改啊 is that gril your friend 改复数 is that a mouse?改复数句 that boy is good.改复数 That is a photo.改复数 this is my cousin 改复数句子 把This is a box and that is a knife.改成复数形式