O livro de história cujo autor conhecemos ontem está na mochila da Júlia.

Cujo means whose. It connects o livro de história to autor and shows a possession relationship: the author is the author of the book.

So the core idea is:

o livro ... cujo autor ...
= the book ... whose author ...

In this sentence, it introduces a relative clause describing which book we are talking about.

It is correct and fully natural, but it sounds more formal or more written than very casual everyday speech.

In conversation, many Brazilians often avoid cujo by rephrasing the sentence. For example, they may change the word order instead of using a whose structure.

So:

• cujo = standard and elegant
• everyday speech = often rephrased in another way

Because cujo agrees with the noun that comes after it, not with the noun before it.

Here the noun after it is autor, which is:

• masculine
• singular

So the correct form is cujo.

The forms are:

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

For example:

• a professora cuja filha mora aqui
• os livros cujos autores são brasileiros

Because standard Portuguese does not use an article right after cujo.

So:

• cujo autor = correct
• cujo o autor = incorrect

This is an important rule. Cujo already contains the possessive idea, so Portuguese does not add o / a / os / as immediately after it.

Not directly.

Que by itself does not express possession, so o livro de história que conhecemos o autor ontem is not standard Portuguese.

If you want to avoid cujo, you usually need to restructure the sentence. A standard alternative is something like:

Conhecemos ontem o autor do livro de história que está na mochila da Júlia.

Another more formal alternative is possible with do qual, but it is heavier and less elegant here.

So in this sentence, cujo is the best standard choice.

Yes, conhecemos can be either:

• we know / we meet in the present, or
• we met / got to know in the preterite

For nós, many verbs have the same form in the present and simple past.

Here, the word ontem makes the meaning clearly past:

• conhecemos ontem = we met yesterday or we got to know yesterday

So context tells you which tense it is.

In this sentence, it most naturally means to meet or to become acquainted with.

That is because of ontem:

• conhecemos o autor ontem = we met the author yesterday

If Portuguese wants to say we know the author, that would usually be understood from context without a time expression like ontem.

So here conhecer is being used in the sense of meeting someone for the first time.

It most naturally refers to conhecemos.

So the intended reading is:

• we met the author yesterday

not:

• the book is in Júlia’s backpack yesterday

Both word order and meaning push you toward attaching ontem to conhecemos.

Because Portuguese normally uses estar for location.

So:

• O livro está na mochila = The book is in the backpack

This is a basic rule:

• ser = identity, classification, permanent-type characteristics
• estar = location, condition, temporary states

Since the sentence is talking about where the book is, está is the correct verb.

There are two contractions here:

• na = em + a
• da = de + a

So:

• está na mochila = is in the backpack
• a mochila da Júlia = Júlia’s backpack

Portuguese commonly shows possession with de:

• a mochila da Júlia = literally the backpack of Júlia

Also, in many varieties of Brazilian Portuguese, people often use a definite article before a person’s name:

• a Júlia

So:

• de + a Júlia → da Júlia

In some regions or styles, you may also hear or see de Júlia.

Portuguese often uses de + noun where English uses a noun as a modifier.

So:

• livro de história = literally book of history
• natural English = history book

This same pattern appears all the time:

• xícara de café = coffee cup
• aula de inglês = English class
• professor de matemática = math teacher

So de história is just the normal Portuguese way to classify the type of book.

Because this relative clause is restrictive: it identifies which book we mean.

The sentence is talking about:

• the history book whose author we met yesterday

That clause is necessary information. It tells us which book.

If you added commas, the clause would sound more like extra, non-essential information:

• O livro de história, cujo autor conhecemos ontem, está na mochila da Júlia.

That version suggests the listener already knows which book is being discussed, and the part about the author is just additional detail.

The main clause is:

O livro de história ... está na mochila da Júlia.

So:

• main subject = O livro de história
• main verb = está

The part cujo autor conhecemos ontem is an embedded relative clause describing the book.

A useful way to parse the sentence is:

• O livro de história = the main noun phrase
• cujo autor conhecemos ontem = a clause modifying livro
• está na mochila da Júlia = the main statement about that book

So even though autor appears in the middle, the whole sentence is still mainly about the book.