friend 、what、 it、this 各为什么词

friend 、what、 it、this 各为什么词 what this about it what is this ?It is a pen而不是It is pen.但为什么what's this? It's what's this?的回答为什么是it's… 为什么不用this is…回答 What's this It's pen .为什么答句不用this is pen,到底it,that ,this 的用法 What is this 是This is a pen.还是.It is a pen?为什么? 英语翻译为什么答案选what color is it 不选 what's this 为什么可以说What's this?而不可以说What's it? what is this?-It is. What（ )this?It's( )apple What is this?It is What is this?It is S.为什么回答不是 It is a S.