Is this her knife变复数

Is this her knife变复数 This is a knife.怎样变复数. this is knife in the box（变复数句 This is her watch.[变复数】 That is her knife.（改为复数） Is this your knife 改为复数句子 Whose knife is this?(改成复数) This is a knife 改为复数 this is my knife改复数句 This is a box and that is a knife.改为复数形式〔怎么变的〕 That is her knife.（改为复数） ____are___ ___ ____ . is this her bike.复数