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? Is this a book 改成复数形式 Those are chairs 改成单数形式 What are these 1 these are your sister 改成复数怎么改 2 they are my familys That is a yellow pineappie改成复数句They are orange改成单数句,The girl This is my friend 和 These are my sist 改成否定句怎么改啊,急... what is this?改复数句 This is a book.改一般疑问句 These are feet 对feet This is a toy bus.改复数 These are some watches on the desk.这个该 把The man is a doctor改成复数句They are all**? What are those? (改成单数形式) These are my shoes.(改成单数形式) It is a Whose teachers are these They are ours改成单数形式 This is a pen ,That is her book怎么改成否定句 These are my sisters怎 Are you tourisits?(改成单数句) Is your friend a tourist,too.(改成复数