英语怎么变复数形式?如：This is a box.要说出怎么是这样变的？

This is a box and that is a knife.改为复数形式〔怎么变的〕 this is a piano怎么变复数 is this that 变复数怎么变 this is a bix.变成复数的句子怎么变 This is my sister?怎么变复数形式?代词my用变复数吗? This is a tomato变复数形式是? this is a box.变复数句___________? Is it your box?变复数形式 This is a pen变为复数句,怎么变 Is this your backpack?变复数要怎么变啊! is怎么变复数 It`s a big heavy box.变成复数怎么变