O caderno está dentro da mochila.

In Portuguese, it is very common to use the definite article with nouns when talking about a specific thing.

So o caderno means the notebook:

• o = the for a masculine singular noun
• caderno = notebook

You can sometimes leave out the article in Portuguese, but in a sentence like this, o caderno sounds natural and standard.

Every noun in Portuguese has grammatical gender, usually masculine or feminine.

Here:

• o caderno is masculine
• a mochila is feminine

This does not mean the objects are biologically male or female. It is just part of the grammar.

A common pattern is:

• nouns ending in -o are often masculine
• nouns ending in -a are often feminine

So:

• caderno → masculine
• mochila → feminine

But this is only a pattern, not a perfect rule, so gender often has to be learned along with the noun.

Portuguese has two main verbs for to be: ser and estar.

Here, está is used because the sentence describes location:

• O caderno está dentro da mochila. = The notebook is inside the backpack.

For location, Portuguese normally uses estar, not ser.

Compare:

• O caderno está na mochila. = The notebook is in the backpack.
• É um caderno novo. = It is a new notebook.

So:

• estar = location, temporary states
• ser = identity, description, permanent characteristics

Dentro da means inside the.

It is made of:

• dentro = inside
• da = of the / from the, but here it is part of the expression

So:

• dentro da mochila = inside the backpack

A very common full form is dentro de:

• dentro de uma mochila = inside a backpack
• dentro do carro = inside the car

In everyday Portuguese, dentro da mochila is very natural.

Because in Portuguese, de + a contracts to da.

So:

• de + a mochila → da mochila

This kind of contraction is very common:

• de + o → do
• de + a → da
• em + a → na
• em + o → no

Examples:

• dentro da mochila = inside the backpack
• dentro do carro = inside the car
• no quarto = in the room
• na bolsa = in the bag

Yes. O caderno está na mochila is completely natural and often more common in everyday speech.

Difference:

• na mochila = in the backpack
• dentro da mochila = inside the backpack

Dentro da is a little more explicit: it emphasizes that the notebook is physically inside the backpack, not just associated with it.

In many situations, both work.

Yes. The correct form is está, with an acute accent on á.

This accent is part of the standard spelling and also helps show the stress:

• es-TÁ

Without the accent, esta is a different word:

• esta mochila = this backpack

So:

• está = is
• esta = this

That is an important difference.

A simple approximate pronunciation for an English speaker is:

oo kah-DER-noh es-TAH DEN-troo dah mo-SHEE-lah

A few notes:

• o in o caderno is often pronounced more like oo
• caderno has stress on der
• está has stress on the last syllable: tá
• mochila sounds like mo-SHEE-la

Pronunciation varies a bit by region, but this is a useful general Brazilian pronunciation.

The normal word order is:

O caderno + está + dentro da mochila

This is the most neutral and natural order:

• subject + verb + place

You can change the order in some contexts, especially for emphasis:

• Dentro da mochila está o caderno.

But that sounds more marked or literary. For everyday speech, the original order is best.

Yes, if you mean a backpack instead of the backpack.

Compare:

• O caderno está dentro da mochila. = The notebook is inside the backpack.
• O caderno está dentro de uma mochila. = The notebook is inside a backpack.

Notice that with uma mochila, you usually say dentro de uma mochila, not dentro da mochila, because da only works with a.

So:

• dentro da mochila = inside the backpack
• dentro de uma mochila = inside a backpack

Yes, much more often than English speakers do.

In English, articles are used regularly, but Portuguese often uses them in places where English might omit them. With ordinary nouns, having the article is very common and natural.

So this sentence sounds normal:

• O caderno está dentro da mochila.

Dropping the articles would sound less natural in most ordinary contexts:

• Caderno está dentro de mochila is not the normal way to say it.

For learners, it is usually safer to include the article when the noun is specific.

It would become:

Os cadernos estão dentro da mochila. = The notebooks are inside the backpack.

Changes:

• o → os for masculine plural
• caderno → cadernos
• está → estão

If both nouns were plural:

• Os cadernos estão dentro das mochilas. = The notebooks are inside the backpacks.

And notice:

• de + as = das