As brasas estão quentes.

Because brasas is a feminine plural noun.

• a brasa = singular feminine
• as brasas = plural feminine

In Portuguese, the definite article has to match the noun's gender and number:

• o = masculine singular
• a = feminine singular
• os = masculine plural
• as = feminine plural

So as brasas means the embers.

The singular is brasa.

• a brasa = the ember
• as brasas = the embers

This is a very regular plural:

• nouns ending in -a often form the plural by adding -s
• brasa → brasas

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

Here, estar is used because hot describes a current condition or state. Temperature is usually treated as a temporary state, so Portuguese normally uses estar:

• As brasas estão quentes. = The embers are hot.
• A sopa está quente. = The soup is hot.

If you used ser here, it would sound unnatural in normal speech.

Because the subject is plural: as brasas.

Portuguese verbs agree with the subject in number:

• A brasa está quente. = The ember is hot.
• As brasas estão quentes. = The embers are hot.

So:

• singular subject → está
• plural subject → estão

Because adjectives in Portuguese usually agree with the noun they describe in number, and often in gender too.

Here the noun is plural, so the adjective must also be plural:

• quente = singular
• quentes = plural

Examples:

• A brasa está quente.
• As brasas estão quentes.

A useful detail: quente has the same form for masculine and feminine in the singular. The difference shows up here only in the plural:

• masculine/feminine singular: quente
• masculine/feminine plural: quentes

Not in the singular.

Some adjectives change for gender:

• frio / fria
• bonito / bonita

But quente does not:

• o carvão quente
• a brasa quente

In the plural, both masculine and feminine use quentes:

• os carvões quentes
• as brasas quentes

Because quentes is not directly attached to the noun inside the noun phrase. It is part of the predicate after the verb estar.

Structure:

• As brasas = subject
• estão = verb
• quentes = predicate adjective

This is similar to English The embers are hot.

You can also place the adjective directly after the noun in a noun phrase:

• as brasas quentes = the hot embers

So compare:

• As brasas estão quentes. = The embers are hot.
• As brasas quentes = the hot embers

Usually, no—not in normal everyday Portuguese.

Portuguese often uses articles where English may or may not use them. In a natural sentence, as brasas sounds much better than brasas by itself.

• natural: As brasas estão quentes.
• unusual in ordinary speech: Brasas estão quentes.

Dropping the article might happen in:

• headlines
• poetry
• very special stylistic contexts

But for a learner, it is best to keep as here.

A rough pronunciation guide is:

az BRA-zas es-TAU KEN-tchis

A few helpful points:

• as is often pronounced like az before a voiced sound such as b in brasas
• br sounds like br in English, but with a Portuguese r
• estão has a nasal ending, something like eh-STAŨ
• in many parts of Brazil, te in quentes sounds like tchi, so KEN-tchis

You do not need perfect pronunciation right away, but it helps to notice that connected speech can change the sound of s.

Not always. Quente can cover both hot and warm, depending on context.

For example:

• O café está quente. could mean The coffee is hot or The coffee is warm, depending on the situation.
• As brasas estão quentes. clearly suggests real heat, so hot is the natural translation.

So the Portuguese word is a little broader than the English word hot in some contexts.

It follows a very common pattern:

definite article + noun + estar + adjective

So here:

• as = definite article
• brasas = noun
• estão = form of estar
• quentes = adjective

You can reuse this pattern with many other words:

• As crianças estão cansadas. = The children are tired.
• As janelas estão abertas. = The windows are open.
• As pedras estão frias. = The stones are cold.

This makes As brasas estão quentes a very useful model sentence for learning agreement and estar.