There are a lot of studentsplaing on the ground now=There are a lot of students who are plaing on the ground now
因为一个句子里只能出现一个主语 一个谓语 一个宾语 ,（并列的算一个,要有连接词如and,or等） who 后面另外成为一个独立定语从句 来修饰 students（宾语）自成一句 所以也要有其独立的主谓宾