第四句话写的不对,所以不做解答.
否定句：
There isn't a pen.
There aren't two brothers.
There isn't a post office.
There isn't a school.
一般疑问句都是把be动词(is,are)提前,回答时如果要肯定回答就是Yes,there + 问句首词,如果否定回答就是No,there + 问句首词 + not：
--- Is there a pen?--- Yes,there is.或No,there isn't.