A professora cujas alunas gostam de estudar na biblioteca vai explicar a lição amanhã.

Cujas is a possessive relative pronoun. It works like English whose.

It connects a professora with alunas and shows possession:

• the teacher
• whose students

So cujas alunas means whose female students.

This is a very useful structure in Portuguese, but it is more formal than many everyday alternatives in spoken Brazilian Portuguese.

Because cujo agrees with the thing being possessed, not with the possessor.

In this sentence:

• possessor: a professora → singular feminine
• possessed thing: alunas → plural feminine

Since alunas is feminine plural, the correct form is cujas.

The full set is:

• cujo = masculine singular
• cuja = feminine singular
• cujos = masculine plural
• cujas = feminine plural

So:

• o professor cujos alunos...
• a professora cujas alunas...

After cujo/cuja/cujos/cujas, Portuguese normally does not use an article.

So you say:

• cujas alunas
• not cujas as alunas

That is a fixed rule of this structure. Since cujo already works as a possessive determiner, adding a/as/o/os would be incorrect.

The subject of gostam is alunas.

So the structure is:

• cujas alunas gostam de estudar na biblioteca

That means the students are the ones who like to study in the library.

This is important because English speakers may briefly wonder whether a professora is the one who likes studying, but grammatically it is alunas.

Because the verb gostar normally requires the preposition de.

So in Portuguese you say:

• gostar de estudar
• gostar de música
• gostar de livros

This is one of those verb + preposition patterns you simply need to learn.

Examples:

• Eu gosto de ler.
• Eles gostam de viajar.
• As alunas gostam de estudar.

Na is a contraction of:

• em
  • a = na

So:

• na biblioteca = in the library

Other common contractions are:

• no = em + o
• nos = em + os
• nas = em + as

This kind of contraction is extremely common in Portuguese.

Here, na biblioteca most naturally goes with estudar.

So the idea is that the students like to study in the library.

It does not naturally mean that the teacher is in the library while explaining the lesson tomorrow. If you wanted that meaning clearly, you would usually place the phrase differently or add more context.

Vai explicar is the ir + infinitive future, which is very common in Brazilian Portuguese.

So:

• vai explicar = is going to explain / will explain

You could also say:

• A professora explicará a lição amanhã.

That version is correct too, but it sounds more formal or more written. In everyday Brazilian Portuguese, vai explicar is often the more natural choice.

Because cujas alunas gostam de estudar na biblioteca is a restrictive relative clause: it identifies which teacher we are talking about.

In other words, it is essential information, not extra information.

Compare the idea in English:

• The teacher whose students like to study in the library... → no comma if it identifies the teacher
• The teacher, whose students like to study in the library, ... → commas if it is extra information

In Portuguese, the sentence as written treats that clause as essential, so there is no comma.

Portuguese often uses the definite article where English may not.

Here, a lição sounds natural because it refers to a specific lesson being explained.

Compare:

• explicar a lição = explain the lesson
• explicar uma lição = explain a lesson

Using a suggests a known or specific lesson, probably one already understood from the context.

Yes. Amanhã is quite flexible.

You can say:

• A professora cujas alunas gostam de estudar na biblioteca vai explicar a lição amanhã.
• Amanhã, a professora cujas alunas gostam de estudar na biblioteca vai explicar a lição.
• A professora vai explicar a lição amanhã.

Putting amanhã at the end is very natural and common, but other positions are possible depending on emphasis.

It is correct and educated, but it can sound somewhat formal in everyday speech.

In writing, formal speech, and careful language, cujo/cuja/cujos/cujas is perfectly normal. In casual spoken Brazilian Portuguese, people often prefer other ways to express the same idea.

Still, learners should know it well because:

• it appears in writing
• it is grammatically important
• it is often tested in classes and exams

Not directly, if you want to keep the same possessive meaning.

Que is a general relative pronoun, but it does not by itself mean whose.

So:

• a professora cujas alunas... = correct for the teacher whose students...

If you replace cujas with que, you would need to restructure the sentence completely, and the result would usually be less elegant.

That is why cujo/cuja/cujos/cujas is the standard way to express this kind of possession inside a relative clause.