1） 在代词what, which, who, none, some, any, more, most, all等词的单复数由其指代的词的单复数决定。
All is right.　(一切顺利。)
All are present.　(所有人都到齐了。)

2） 集体名词作主语时，谓语的数要根据主语的意思来决定。如family, audience, crew, crowd, class, company, committee等词后用复数形式时，意为这个集体中的各个成员，用单数时表示该个集体。
His family isn't very large.　他家不是一个大家庭。
His family are music lovers.　他的家人都是音乐爱好者。
但集合名词people, police, cattle, poultry等在任何情况下都用复数形式。
Are there any police around?

3）有些名词，如variety, number, population, proportion, majority 等有时看作单数，有时看作复数。
A number of +名词复数+复数动词。
The number of +名词复数+单数动词。
A number of books have lent out.
The majority of the students like English.