Learn Hardcore Portuguese (Brazil): As alunas cujos cadernos ficaram na sala vão buscá-los amanhã. - The female students whose notebooks were left in the classroom are going to get them tomorrow.

Breakdown of As alunas cujos cadernos ficaram na sala vão buscá-los amanhã.

Why is it as alunas and not os alunos or as alunos?

Because alunas is a feminine plural noun meaning female students.

The article has to match the noun in gender and number:

So:

as alunas = correct
os alunos would mean the male students or the students in a mixed group
as alunos is incorrect because the article and noun do not match

What does cujos mean here?

Cujos is a possessive relative pronoun. It means something like whose.

So:

as alunas cujos cadernos ficaram na sala
= the female students whose notebooks were left in the classroom

It connects alunas and cadernos and shows possession: the notebooks belong to the students.

This word is more formal and much more common in writing than in everyday speech.

Why is it cujos cadernos and not cujas cadernos?