这是一个There be句型.由于一句子中只能有一个主谓宾结构,所以后面只能用分词来补充说明,而不能用work.要用的话必须有一个连词,可以改成这样:
There are two hundred doctors and nurses and they are working hard in the hospital.
There are two hundred doctors and nurses working hard in the hospital这句话中working working hard in the hospital 是现在分词短语来修饰前面的doctors and nurses.相当于一个定语从句,此句也可以改写成:
There are two hundred doctors and nurses who are working hard in the hospital.