Learn Hardcore Portuguese (Brazil): Na sala, temos um sofá confortável e um tapete pequeno. - In the living room, we have a comfortable sofa and a small rug.

em a sala → na sala (correct)
em sala (without the article) is also possible but sounds more generic, like in a room / in class, not a specific room of the house.

You normally use the contraction (na / no) when you mean a specific place with the.

Could I say Na sala de estar instead of Na sala for in the living room?

Yes.

Na sala = in the room, and in a house context it is often understood as in the living room.
Na sala de estar = literally in the living room, more explicit and a bit more formal or descriptive.

Both are correct. In everyday speech, Brazilians often just say na sala when they mean the living room.

Why is there a comma after Na sala?

The phrase Na sala is placed at the beginning of the sentence to set the location (a prepositional phrase). In both Portuguese and English, when you move such a phrase to the front, it is usually followed by a comma:

Na sala, temos … = In the living room, we have …

You could also say:

Temos um sofá confortável e um tapete pequeno na sala.

Both word orders are correct; the comma is just marking that Na sala is an introductory element.

Why is temos used here, and not há or tem?

Temos is the we have form of ter:

ter = to have
nós temos = we have

So Na sala, temos um sofá… is literally In the living room, we have a sofa…

Other options:

Na sala, há um sofá… → In the living room, there is a sofa…
- há is more neutral/formal and does not say who owns the items.
Na sala, tem um sofá… → In Brazil, often used like there is a sofa in the living room.
- This sounds informal / conversational and is very common in spoken Brazilian Portuguese.

In your sentence, temos highlights that we have these things in the room.

Could I say Na sala, tem um sofá confortável e um tapete pequeno? Does that sound natural?

Yes, that is very natural in Brazilian Portuguese, especially in speech.

Na sala, tem um sofá confortável e um tapete pequeno.
≈ In the living room, there is a comfortable sofa and a small rug.

Some teachers consider impersonal tem (meaning there is / there are) to be informal, and prefer há in writing:

Na sala, há um sofá confortável e um tapete pequeno.

But in everyday Brazilian speech, tem in this sense is extremely common and sounds perfectly natural.

Why is there no nós before temos? How do I know it means we?

Portuguese often drops subject pronouns because the verb ending shows who the subject is.

eu tenho (I have)
tu tens (you have – rare in Brazil)
ele / ela / você tem (he / she / you have)
nós temos (we have)
eles / elas / vocês têm (they / you all have)

In temos, the ending -mos clearly marks we. So Na sala, temos… is understood as In the living room, we have…, even without nós.

You could say Na sala, nós temos…, but in normal speech people usually omit nós unless they want to emphasize we.

Why is it um sofá and not uma sofá?

Because sofá is a masculine noun in Portuguese.

Masculine singular: um sofá
Feminine singular: uma cadeira (a chair), uma mesa (a table)

Gender in Portuguese is mostly grammatical and arbitrary; you just learn each noun with its article:

o sofá – the sofa (masculine)
um sofá – a sofa (masculine)
o tapete – the rug / carpet (masculine)
um tapete – a rug / carpet (masculine)

Since both sofá and tapete are masculine, they take um.

Why does confortável look the same for sofá and for feminine nouns? Doesn’t it change?

Confortável is one of the adjectives that has the same form for masculine and feminine in the singular.

Masculine singular: um sofá confortável
Feminine singular: uma cadeira confortável

It only changes in the plural:

Masculine plural: sofás confortáveis
Feminine plural: cadeiras confortáveis

So the pattern is:

singular: confortável
plural: confortáveis

Many adjectives ending in -vel, -al, -ar, -or, -ês, etc. do not change between masculine and feminine in the singular.

Why is the adjective after the noun in um sofá confortável, not before like in English?

In Portuguese, the default position for most adjectives is after the noun:

um sofá confortável = a comfortable sofa
um carro novo = a new car
uma casa grande = a big house

You can sometimes put adjectives before the noun, but that usually adds emphasis or changes the nuance a bit (often more subjective, emotional, or poetic):

um confortável sofá – stylistic, more literary / poetic, sounds unusual in everyday speech.

So in normal, neutral speech, sofá confortável (noun + adjective) is the standard order.

Could I also say um pequeno tapete instead of um tapete pequeno? Is there a difference?

Yes, both are grammatically correct:

um tapete pequeno – neutral, descriptive, standard
um pequeno tapete – a bit more emphatic / subjective, or slightly more formal / literary

Putting pequeno before the noun can:

make it sound more like an evaluation or highlight the smallness,
feel more stylistic, like a little rug rather than just a small rug.

In everyday neutral speech, um tapete pequeno is more common.

What exactly does tapete mean? Is it a rug or a carpet?

Tapete usually means rug or small carpet, something you can roll up and move.

There is also:

carpete – used in Brazil for wall-to-wall carpet or carpeted floor.

So:

um tapete pequeno – a small rug (like an area rug or mat)
um carpete – carpet covering the floor.

In many everyday situations, people may still say tapete even when it is fairly big, as long as it is a separate piece and not fixed to the floor.

How do adjectives agree with nouns in this sentence, and how would it look in the plural?

In Portuguese, adjectives agree with the noun in gender (masculine / feminine) and number (singular / plural).

In your sentence:

um sofá confortável
- sofá: masculine singular
- confortável: masculine singular form
um tapete pequeno
- tapete: masculine singular
- pequeno: masculine singular (ends in -o)

If everything were plural:

Na sala, temos sofás confortáveis e tapetes pequenos.
- sofás (m. plural) → confortáveis (plural)
- tapetes (m. plural) → pequenos (plural)

So you change:

confortável → confortáveis in the plural
pequeno → pequenos / pequenas depending on gender and number.