Learn Hardcore Portuguese (Portugal): O meu quarto fica no rés do chão.

This use of the article is standard in Portugal. Leaving it out is possible in some contexts, but in neutral sentences like this one, o meu quarto is the normal, idiomatic choice in European Portuguese.

Can I also say Meu quarto fica no rés do chão without o?

In European Portuguese, that sounds a bit colloquial or Brazilian-like, but it is not impossible. You might hear it in very informal speech, but:

In Portugal, the most natural everyday form is O meu quarto fica…

In Brazilian Portuguese, dropping the article is more common and natural:

Meu quarto fica no térreo. (Brazil)

So, if your target is Portuguese from Portugal, learn and prefer o meu quarto.

What exactly does fica mean here? Why not é or está?

Ficar has several meanings, but here it means to be located / to be situated.

O meu quarto fica no rés do chão.
= My room is on the ground floor / My room is located on the ground floor.

Using é here (O meu quarto é no rés do chão) is not idiomatic for location of a room.
Using está (O meu quarto está no rés do chão) can occur, but for general, neutral locations of buildings, rooms, places, ficar is the most natural choice in European Portuguese.

So you can think:

ser – what something is (identity, essential characteristics)
estar – how/where something is (state, temporary situation)
ficar – where something is situated / ends up (location, result)

What verb tense and person is fica? How would it change with other subjects?

Fica is the 3rd person singular of ficar in the present tense (presente do indicativo).

Brief present tense of ficar:

eu fico – I stay / I am located
tu ficas – you stay (singular, informal in Portugal)
ele / ela / você fica – he / she / you stay
nós ficamos – we stay
vocês ficam – you (plural) stay
eles / elas ficam – they stay

In the sentence:

Subject: o meu quarto (3rd person singular)
Verb form: fica

What does no mean in no rés do chão?

No is a contraction of the preposition em (in / on / at) + the definite article o (the, masculine singular):

em + o = no
em + a = na
em + os = nos
em + as = nas

So:

no rés do chão = em + o rés do chão
→ literally: in the ground floor
→ natural English: on the ground floor / at ground-floor level

Why is it rés do chão and not something like rés de chão?

Do is a contraction of de + o:

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

Inside the expression:

rés – a noun meaning something like level with / flush with
chão – the ground, masculine singular

So:

rés do chão = rés de + o chão
→ literally: level of the ground
→ idiomatically: ground floor (same level as the street).

What exactly does rés do chão mean? Is it the same as “first floor”?

No, it is not the same as “first floor” in British English.

In Portugal:

rés do chão = ground floor, street level
primeiro andar = first floor above the ground floor

So:

British English:
- ground floor → rés do chão
- first floor (above ground) → primeiro andar
American English:
- first floor (street level) → rés do chão
- second floor → primeiro andar

So O meu quarto fica no rés do chão = My room is on the ground floor.

Is rés do chão only used in Portugal? What would Brazilians say?

Rés do chão is mainly a European Portuguese expression.

In Brazilian Portuguese, people usually say:

térreo or andar térreo
- Meu quarto fica no térreo.

They may also say:

andar de baixo (more informal: the floor downstairs).

So:

Portugal: no rés do chão
Brazil: no térreo / no andar térreo / no andar de baixo

What is the structure inside rés do chão? What are the genders?

Breakdown:

rés – masculine noun (an old word related to level, flush with).
chão – masculine noun meaning ground / floor (the actual surface of the ground, not a storey of a building).

The articles and contractions match this:

o rés – the ground level (masculine singular)
o chão – the ground (masculine singular)
do chão = de + o chão

So:

no rés do chão = em + o rés de + o chão → contracted to no rés do chão.

Is chão masculine or feminine? How can I guess?

Chão is masculine:

o chão – the ground / the floor (surface)
do chão – of the ground
no chão – on the ground

A rough pattern: nouns ending in -ão are usually masculine in Portuguese (though there are exceptions, like a mão).

In rés do chão, we see this masculinity in:

do (= de + o)
because the article o is masculine singular.

Could I put the words in a different order, like No rés do chão fica o meu quarto?

Yes, that is possible in Portuguese. Both are grammatically correct:

O meu quarto fica no rés do chão. – neutral, most common order.
No rés do chão fica o meu quarto. – puts slight emphasis on the location (“On the ground floor is my room”).

The usual neutral pattern is:

[Subject] + [Verb] + [Place]
→ O meu quarto
- fica
  - no rés do chão

Changing to [Place] + [Verb] + [Subject] is often used for emphasis or in more literary or descriptive styles.

How do you pronounce quarto and rés do chão in European Portuguese?

Approximate European Portuguese pronunciation:

quarto – /ˈkwaɾ.tu/
- like KWAHR-too, with a tapped r (like the Spanish r in pero).
rés do chão – roughly /ˈʁɛʃ du ˈʃɐ̃w̃/
- rés: like RESH, with a short open e (similar to e in red)
- do: like doo, but short
- chão: like SHAUN, nasal at the end

Very rough English approximations:

quarto ≈ KWAHR-too
rés do chão ≈ RESH du SHAUN (with the final n mostly nasal, not fully pronounced as an English n).

Does quarto always mean “bedroom” in Portuguese?

Quarto most commonly means bedroom:

o meu quarto – my bedroom
um quarto de hotel – a hotel room

It can also mean fourth (the number/ordinal), but then the context makes it clear:

o quarto andar – the fourth floor
a quarta página / a página quatro – the fourth page

In O meu quarto fica no rés do chão, the context clearly tells you it is bedroom, not fourth.