为什么有的集合名词后面加“is”有的又用“are”.有什么规律吗?

为什么有的集合名词后面加“is”有的又用“are”.有什么规律吗?

集合名词作主语时,主谓一致关系是一个较为复杂的问题.对此类问题我们可以从“数”的角度分为四类.
1)单数—复数型.凡是有复数词尾变化形式的集合名词都属于此类.如：a class—classes; a family—families; a government—governments; an army—armies; a people—peoples; a group—groups; a crowd—crowds; a crew—crews等.这类集合名词强调的是整体性,即当作一个整体或多个整体来看待.属于这类集合名词的单数作主语时,谓语动词用单数；复数形式作主语时,谓语动词用复数.
【例如】
A big crowd often gathers on the square every morning.
The government has decided to pass the bill.
There are huge crowds in the streets on Sunday.
There are many English-speaking peoples in the world.
但应注意,这类集合名词的单数形式有时表示复数概念,所以这些集合名词的单数形式也可归为“单复同形型”中.
2)单数型.这类集合名词表示的是人或事物的整体,即把这类人或事物的全部包括在内,所以只有单数形式.如作主语,谓语动词常用单数.这类名词常见的有：humanity,mankind,proletariat等.
【例如】The proletariat is the greatest class in the history of mankind.In the fields of production and scientific experiment,mankind makes constant progress.
3)复数型.这类集合名词在形式和内容上是相互矛盾的,就是说它们只有单数形式,但表达的都是复数概念.它强调的是集体中的个体性.这类名词有：police,cattle,faculty,flock,machinery,vermin,personnel等.它们作主语时,谓语动词要用复数.
【例如】The police have caught the murder.
Our personnel are very highly trained.
The vermin are very dangerous.
4)单复同形型.这类集合名词的单数形式既可表示单数也可表示复数.作主语时,用单数动词或复数动词均可,有时意义区别不大.
【例如】The school teaching staff are (is) excellent.
The public is (are) requested not to litter in the park.
The teaching profession claim(s) to be badly paid.
这类集合名词常见的有：class,family,team,crew,board,herd,committee,party,jury,enemy,audience等.
根据说话人的心理意向若把这个集合名词所代表的人或事物看作一个整体,就认为是单数,用单数动词；若把它所代表的人或事物看作若干个个体的话,就认为其为复数,用复数动词.
试比较：The football team is playing well.
那个足球队打得非常漂亮.
The football team areshavingsbath and are then coming back here for tea.
足球队员们正在洗澡,然后来这里吃茶点.
The family is a very happy one.
那个家庭是一个非常幸福的家庭.
That family are very pleased about the news of William's success.
全家人对威廉的成功都感到很高兴.