1.This is a Chinese stamp.(改为否定句和一般疑问句）
否定句：There isn't a Chinese stamp.
一般疑问句：Is there a Chinese stamp?
2.These are Chinese stmps..(改为否定句和一般疑问句）
否定句：There aren't Chinese stamps.
一般疑问句：Are there Chinese stamps?
希望能够帮到楼主