为什么this is a XXX改复数,有时改成these are 有时又改成they are?

为什么this is a XXX改复数,有时改成these are 有时又改成they are? This is a garden改复数 this is a ruler.改复数 Is this a plane?yes,it is 改复数句 is this a boy or a girl改复数句 This is a bottle of milk.怎样改写成复数? is this a car?改为复数形式怎么改? This is a backpack(改为复数句子)怎么改? This is a book(改复数) _______ _______ bookThis is a book(改复数) _______ _______ books this is an orange改复数句 this is my cousin 改复数句子 this is an eraser 改复数 This dress is mine改复数句 this is my knife改复数句 That ruler is a (rectangle) 括号提问 This is a red strawberry 改复数 This is an apple.that is a pen.改复数形式 改复数句如this gift is for youhe is a policemam This is a desk 复数