there are seven days in a week

time作主语谓语动词用单数,因为time 不可数。这句中主语是day,是可数名词,seven days 谓语动词用复数

there be句型真正的主语是be后面的名词,这里days是复数,故be用are

There be 句型中，谓语动词be的单复数形式要看be后第一项内容，这里是可数名词复数(seven) days，所以be要用复数形式。

又如：There is a book on the table.
There are two books on the table.
There is a book, two pens and three pencils on the table.
There are two pens, three pencils and a book on the table.

当名词在be动词前，要整体考虑；当名词在be动词后，则要采取就近原则。
因为在there be句型遵循就近原则，seven days 是复数，所以用are
如：(1)There is a pen and some books
(2)There are some books and a pen
现在明白了吧

在这里用单,复数都可以。
表示（金钱，时间，价格或度量衡的复合名词）作主语 时，通常把这些名词看作一个整体，谓语一般用单数。
但是用复数也可，意思不变。

There are seven days.
Seven days are there.

Seven days相当于主语，所有谓语动词用复数.