Questions & Answers about Их семеро, и они очень сплочённые.
Why is the genitive pronoun Их used here with семеро instead of just saying Они семь?
This is the standard way to express 'there are [number] of us/you/them' in Russian. You use the genitive of the personal pronoun (here, их) followed by a collective numeral (like семеро for seven). It treats the numeral as the predicate, meaning 'Of them, there is a group of seven.'
Does семеро follow the same rules as normal numbers, like requiring the genitive case after it?
In this specific construction where the numeral is the predicate, the pronoun comes first in the genitive (их), and the collective numeral (семеро) is in the nominative. You do not add a noun after it here because the pronoun already tells us who makes up the group.
Where does the adjective сплочённые come from?
It comes from the past passive participle of the verb сплотить (to unite or rally together). In everyday language, it acts as a standard adjective meaning 'close-knit', 'cohesive', or 'tightly united', which is perfect for describing a solid group of people.
Why is сплочённые in the nominative plural if the first half of the sentence used the genitive Их?
The conjunction и connects two independent clauses. The first is the construction Их семеро. The second clause starts fresh with the nominative subject они, so the predicate adjective сплочённые must agree with они in the nominative plural.