This isn't a pen.
These are pens.
Are these pens?/ Is this a pen?
Yes,they are./Yes,it is.
如有问题,欢迎继续追问：）哪个否定句，哪个单复数转换，哪个一般疑问句或提问和肯定句回答。否定句This isn't a pen.单复数转换These are pens.一般疑问句提问Are these pens?/ Is this a pen?肯定句回答Yes, they are. /Yes, it is.