A luz do banheiro está muito forte.

Because Portuguese normally contracts de + o into do.

• de = of / from
• o = the
• do = of the / from the

So a luz do banheiro literally means the light of the bathroom, which in natural English becomes the bathroom light.

Portuguese usually expresses this kind of relationship with de: X de Y = X of Y.

So:

• a luz do banheiro = the light of the bathroom
• natural English: the bathroom light

Using possession with an apostrophe-like idea is much less common here. Portuguese strongly prefers the de structure for things like rooms, objects, and parts of places.

Because estar is used for a current condition or state, while ser is used more for identity or permanent characteristics.

Here, the sentence is talking about how the light is right now: it feels too intense/bright at the moment.

• A luz do banheiro está muito forte. = The bathroom light is very bright/strong.
• é muito forte would sound more like saying that the light is inherently, generally, or characteristically strong.

In everyday speech, estar is the natural choice here.

Forte does basically mean strong, but in Portuguese it is also used for things that are intense, including:

• luz forte = strong/bright light
• som forte = loud sound
• cheiro forte = strong smell
• sabor forte = strong flavor

So with luz, forte is often best translated as bright or harsh, depending on context.

Because muito is acting as an adverb here, meaning very.

• muito forte = very strong / very bright

This is similar to English word order:

• very bright
• muito forte

If muito were being used as an adjective, it would change for gender and number, but here it does not, because it is modifying forte, not a noun.

Portuguese often uses definite articles more than English does.

In this sentence, a luz refers to a specific light: the bathroom light. So using a is the normal, natural choice.

Dropping the article would sound less natural in a standard sentence like this.

Some adjectives change form for gender, but forte does not in the singular.

So you get:

• a luz forte
• o som forte

Both use forte.

Only the plural changes:

• a luz forte
• as luzes fortes

So forte is one of those adjectives that has the same singular form for masculine and feminine.

Yes, people would understand you, and in some contexts it may sound acceptable. But está muito forte is usually more natural if you are commenting on how the light feels at that moment.

A good rough distinction is:

• está muito forte = it seems very bright right now
• é muito forte = it is a very strong light in general

So estar is the more common everyday choice for this sentence.

They mean different things.

• a luz do banheiro = the bathroom light / the light belonging to the bathroom
• a luz no banheiro = the light in the bathroom

Sometimes both could refer to the same real-world thing, but grammatically they are different:

• do banheiro identifies which light
• no banheiro tells you where the light is

Also, no is a contraction of em + o.

In Brazilian Portuguese, banheiro usually means bathroom. Depending on context, it can also correspond to restroom or toilet in English.

So in this sentence, banheiro most naturally means bathroom.

In Brazilian Portuguese, banheiro is approximately pronounced like bahn-YAY-roh.

A few helpful notes:

• nh sounds like the ny in canyon
• ei often sounds like ay
• the final o in Brazilian Portuguese often sounds like oo or a very light oh, depending on accent

So banheiro is roughly bah-NYAY-roo in many Brazilian accents.

Luz is pronounced roughly like looz, but with a Brazilian z sound at the end.

So:

• luz ≈ looz

It is a short one-syllable word.

It is neutral and completely natural in everyday Brazilian Portuguese.

You could say it in normal conversation at home, in a hotel, at a friend’s house, or when asking someone to adjust the light. It is neither especially formal nor especially slangy.