Quando a bateria dos fones sem fios acaba, continuo a estudar em silêncio.

Dos is a contraction of de + os (of + the).

• a bateria dos fones sem fios literally = the battery of the wireless headphones
• So dos expresses possession or relationship, like English “of the” or sometimes the possessive “the headphones’ battery”.

Structure:

• a bateria = the battery
• de + os fones sem fios = of the wireless headphones → dos fones sem fios

The subject of the verb is a bateria, which is singular, not os fones.

The structure is:

• Quando a bateria dos fones sem fios acaba, …
  
  • a bateria = grammatical subject (singular)
  • dos fones sem fios = a prepositional phrase describing bateria

Because bateria is singular, the verb must be:

• ela (a bateria) acaba → acaba (3rd person singular)

You cannot agree the verb with fones, because fones is not the subject here.

Yes, that is correct and natural. Both are possible:

• Quando a bateria dos fones sem fios acaba, continuo a estudar em silêncio.
• Quando acaba a bateria dos fones sem fios, continuo a estudar em silêncio.

Differences:

• Quando a bateria … acaba: subject comes before the verb (more neutral order).
• Quando acaba a bateria …: verb comes first, often used in spoken Portuguese and can slightly emphasise the event (the battery running out) rather than the battery itself.

Grammatically, both are fine.

In Portuguese, the present indicative is very often used in “quando” clauses to talk about general habits or repeated situations, even when in English we might think about the future.

• Quando a bateria dos fones sem fios acaba, continuo a estudar em silêncio.
  = Whenever / every time the battery runs out, I keep studying in silence.

You would normally not say quando a bateria acabar here, because quando + future subjunctive describes a single future event or a more specific future situation. For example:

• Quando a bateria acabar, digo-te.
  = When the battery runs out (that time in the future), I’ll tell you.

In your sentence, the person is describing a regular habit, so the present (acaba) is the natural choice.

In European Portuguese, the most natural way to express a continuing action after continuar is:

• continuar a + infinitive

So:

• continuo a estudar = I continue to study / I keep studying

Details:

• continuo estudar – incorrect. You need the preposition a.
• continuo estudando – sounds Brazilian. In European Portuguese, gerunds like this are used much less, especially with continuar.

So, for Portugal, you should prefer:

• continuo a estudar
• continuamos a trabalhar
• ele continua a ler

Yes, continuo a estudar silenciosamente is grammatically correct, but em silêncio is:

• more idiomatic,
• more common in everyday speech,
• and a bit more neutral or natural-sounding.

Nuance:

• em silêncio = “in silence”, focusing on the state (there is no sound).
• silenciosamente = “silently”, an adverb describing how you study; it can sound a bit more formal or literary, though it’s not wrong.

In most everyday contexts, em silêncio is the better choice.

Em often introduces a state or condition in Portuguese.

• em silêncio = in silence
• em pé = standing (literally “on foot”)
• em casa = at home

You could occasionally see alternatives like:

• no silêncio = in the silence (more specific, with a definite article, and a bit more poetic or descriptive)

But in this sentence, em silêncio is the standard, idiomatic way to say “in silence / quietly (without speaking or making noise)”.

In standard written Portuguese, you do usually write a comma when a dependent clause like a quando-clause comes at the beginning:

• Quando a bateria dos fones sem fios acaba, continuo a estudar em silêncio.

If you invert the order and put the quando-clause second, the comma is usually omitted:

• Continuo a estudar em silêncio quando a bateria dos fones sem fios acaba.

So:

• Clause at the start → normally with comma.
• Clause at the end → normally without comma.

Yes, that sentence is also correct and sounds natural.

Meaning is the same; the difference is focus and rhythm:

• Quando a bateria dos fones sem fios acaba, continuo a estudar em silêncio.
  
  • Slight emphasis on the condition (when the battery dies).
• Continuo a estudar em silêncio quando a bateria dos fones sem fios acaba.
  
  • Slight emphasis on the habit (continuing to study in silence).

Both are fully acceptable in European Portuguese.

Fones sem fios literally means “phones without wires” → wireless headphones/earphones.

• fones = headphones / earphones (informal)
• sem fios = without wires → wireless

In Portugal, people may also say:

• auscultadores sem fios – more formal or technical
• phones sem fios – also heard, influenced by English

In Brazil, a very common equivalent is:

• fone(s) de ouvido sem fio

So fones sem fios is natural in European Portuguese, especially in everyday, informal speech.

Portuguese usually refers to headphones/earphones in the plural, because there are two earpieces:

• os fones / os auscultadores = the headphones

So:

• os fones sem fios = the wireless headphones / earphones

You would only use the singular um fone if you are really talking about one single earpiece, which is less common in general descriptions. In your context, plural is exactly what you want.

You need the article here. Quando bateria dos fones sem fios acaba is incorrect.

In Portuguese, the definite article (o, a, os, as) is used more often than in English, especially with specific, known things:

• a bateria = the battery (in these particular headphones)

In this sentence, you’re talking about the specific battery of your headphones, not just any random battery, so the definite article a is required:

• Quando a bateria dos fones sem fios acaba, … ✔️
• Quando bateria dos fones sem fios acaba, … ✖️ (incorrect)

In European Portuguese, bateria is pronounced roughly:

• [ba-te-RI-a], with the stress on the “ri” syllable.

Details:

• ba – like “ba” in “bat” (but shorter/cleaner)
• te – like “te” in “ten” (but shorter, and t is clear, not “ch”)
• RI – stressed syllable; r is a strong guttural sound in most European accents, like a French or German r
• a – like the final “a” in Italian “casa”

So the stress pattern is:

• ba-te-RI-a