区别是this is 后面只能跟单数,如an appe ,a book。
而there are后面必须跟复数,如 two books,three apples,many apples。
this is 一个+可数名词的单数，these are 一些+可数名词的复数+s
these are：这些都是
These are这些是
These days are past这些日子也会过去的
these postcard are great这些都是伟大明信片
this is：这是
This is这是
this field is optional可选项
This romeo is bleeding罗密欧在流血