为什么要用it is?

为什么要用it is? Is this your sister 答语为什么要用it is. Is that your mother ,Dave Yes ,it is.为什么要用it 呢 Is this/that your sister 的答语是Yes,it is./ No,it isn’t 为什么要用it it is difficult for me.为什么要用for? is't it a lovely weather为什么要用a It is time we went.为什么要用虚拟语气? 为什么还能写成It is time we should go Is this you pencil 为什么不能回答 yes,this is.而是要用Yes,it is. What is this?回答：这是V.为什么不能用：It is a V.而要用:It is V. is that提问为什么用it回答 it is allowed!为什么? here is a card,please fill it out.为什么要用fill it out?