复数句就是主语是复数。单数句就是主语是单数。
例如：Three apples are on the table.
单数句： An apple is on the table
复数句就是做主语和宾语的名词,代词都用复数表达的句子.如：
This is a book .这是单数句,因为 this,a,book 指的都是单数.
改成复数句就是：
Thses are some books.these 是this 的复数 some books 是a book 的复数
There is a child.这是单数句 There are some children.
要注意因为主语改成了复数,所以谓语动词或be 动词也要变化,如：
He does his homework every day.改成复数句：
They do their homework everyday.(homework 是不可数名词,所以不变
