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+sthese 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