Learn Hardcore Portuguese (Portugal): A minha mochila está ao lado da cadeira.

In European Portuguese, it is very common (almost default) to use a definite article before a possessive: a minha mochila, o meu livro, os meus pais, etc.

So:

A minha mochila = my backpack
Minha mochila (without a) is possible, but it sounds marked, more emotional or poetic, or is used in some fixed expressions.

In everyday neutral speech in Portugal, you normally include the article: A minha mochila…

Why is mochila feminine, and how can I tell?

Nouns in Portuguese have grammatical gender, which is often but not always predictable. Mochila ends in -a, and many nouns ending in -a are feminine.

You can see it’s feminine because:

It uses the feminine article a: a mochila
The possessive is feminine: minha instead of meu

So:

a mochila (feminine)
o livro (masculine)

You mainly have to learn noun genders as you go, but endings (-a vs -o) often give you a good guess.

Why is it está and not é?

Portuguese, like Spanish, has two verbs for to be: ser and estar.

estar is used for location, temporary states, and conditions.
ser is used for identity, inherent characteristics, time, origin, etc.

Location almost always uses estar:

A minha mochila está ao lado da cadeira. – My backpack is next to the chair.
Ela está em casa. – She is at home.

Using é here (A minha mochila é ao lado da cadeira) would be incorrect.

What exactly does ao mean in ao lado?

Ao is a contraction of the preposition a + the masculine singular article o.

So:

a
- o → ao

Literally, ao lado = a o lado = at the side / to the side.
In modern English we just say next to or beside, but the idea is “at the side of” something.

Why is it ao lado da cadeira and not ao lado de a cadeira?

Portuguese contracts certain prepositions with the definite articles:

de + a → da
de + o → do
de + as → das
de + os → dos

So:

ao lado de a cadeira becomes ao lado da cadeira
Literally: at the side of the chair

De a without contraction (de a cadeira) is ungrammatical in normal modern Portuguese.

What is the difference between ao lado de and perto de?

Both can be translated as “near” in English, but they’re not the same:

ao lado de = next to / beside → very close, right by the object, normally on one side of it.
- A mochila está ao lado da cadeira. – The backpack is right next to the chair.
perto de = near / close to → nearby, but not necessarily right next to it.
- A mochila está perto da cadeira. – The backpack is near the chair (maybe a little distance away).

So ao lado de is more specific and closer than perto de.

Why is it da cadeira and not de cadeira?

De cadeira (without an article) would sound like a general description, not a specific chair.

da cadeira = de + a cadeira → of the chair / from the chair / by the chair (a specific chair already known from context).

If you wanted to mean “next to a chair” (not a specific one), you’d say:

A minha mochila está ao lado de uma cadeira. – My backpack is next to a chair.

Could I say A mochila minha está ao lado da cadeira?

This word order is not natural in modern European Portuguese.

Normally:

A minha mochila (article + possessive + noun)

Structures like a mochila minha can appear in poetry or special emphatic contexts, but in everyday speech you should stick with:

A minha mochila está ao lado da cadeira.

Can I leave out minha and just say A mochila está ao lado da cadeira?

Yes, but the meaning changes:

A minha mochila está ao lado da cadeira. – My backpack is next to the chair.
A mochila está ao lado da cadeira. – The backpack is next to the chair. (not necessarily clear whose backpack it is)

If context already makes it obvious that you’re talking about your backpack, the shorter version can be fine. Otherwise, use minha to make ownership clear.

Can I omit está and just say A minha mochila ao lado da cadeira?

No. In Portuguese, you normally need the verb estar (or ser) to form a complete sentence like this.

So you must say:

A minha mochila está ao lado da cadeira.

Without está, it sounds like a fragment, not a full sentence.

How would I say this in the plural, for example “My backpacks are next to the chairs”?

You need to make the article, possessive, nouns, and sometimes the article after de all agree in number:

As minhas mochilas estão ao lado das cadeiras.

Breakdown:

as – feminine plural article
minhas – feminine plural possessive
mochilas – plural noun
estão – 3rd person plural of estar
das cadeiras – de + as cadeiras (of the chairs)

Is this sentence the same in Brazilian Portuguese, or does it change?

The sentence A minha mochila está ao lado da cadeira. is perfectly correct and natural in both European and Brazilian Portuguese.

The main difference is that Brazilians more often drop the article with possessives in speech:

Minha mochila está ao lado da cadeira.

In Portugal, including the article a is more standard:

A minha mochila está ao lado da cadeira.

Aside from that stylistic difference, the structure and meaning are the same.