Os alunos não falam alto durante a prova.

In Portuguese, the definite article is used more often than in English. Os alunos means the students, and it sounds very natural when talking about a group in general or a specific known group, such as students in a class.

Just alunos is possible in some contexts, but os alunos is the most natural choice here.

Because alunos is a masculine plural noun.

• o aluno = the male student / the student
• os alunos = the students

If the group were only female, you would say:

• a aluna = the female student
• as alunas = the female students

If the group is mixed, Portuguese normally uses the masculine plural: os alunos.

In Portuguese, não usually goes directly before the verb it negates.

So:

• falam = they speak
• não falam = they do not speak

This is the normal way to form negation in Portuguese.

Falam is the 3rd person plural of the present tense of falar.

The present tense here can express:

• a general truth
• a habit
• a rule
• something happening in the present

So Os alunos não falam alto durante a prova can mean something like a general rule or normal behavior: The students do not speak loudly during the test.

Conjugation of falar in the present:

• eu falo
• você/ele/ela fala
• nós falamos
• vocês/eles/elas falam

In Portuguese, some adjectives can be used like adverbs, especially after verbs of speaking, singing, and similar actions.

So:

• falar alto = to speak loudly
• falar baixo = to speak quietly/softly

This is very common and natural.

Using altamente here would sound wrong. Altamente usually means something more like highly, not loudly.

No. Here alto is not describing alunos. It is describing how they speak, so it functions adverbially.

That is why it stays alto, not altos.

Compare:

• Os alunos são altos. = The students are tall.
  Here altos agrees with alunos.

• Os alunos falam alto. = The students speak loudly.
  Here alto describes the manner of speaking, so it does not change.

Yes. Falar em voz alta is another very common way to say to speak out loud / to speak loudly.

So these are both natural:

• Os alunos não falam alto durante a prova.
• Os alunos não falam em voz alta durante a prova.

The first is shorter and very common. The second is slightly more explicit.

Durante means during.

So:

• durante a prova = during the test

It introduces a time period in which something happens or does not happen.

Examples:

• Durante a aula = during class
• Durante o filme = during the movie
• Durante a reunião = during the meeting

Prova is a feminine noun, so it takes the feminine singular article a.

• a prova = the test / the exam

That is why you get:

• durante a prova

not durante o prova.

It can mean both, depending on context.

In this sentence, a prova clearly means the test or the exam, because of the school context with os alunos.

Common meanings:

• prova = test, exam
• prova = proof, evidence

Context tells you which meaning is intended.

Portuguese often leaves out subject pronouns because the verb ending already shows who the subject is.

• falam already tells you the subject is they
• and os alunos is already stated, so eles is unnecessary

So:

• Os alunos não falam alto durante a prova. = perfectly natural
• Os alunos, eles não falam alto durante a prova. = possible for emphasis, but not needed here

Yes, a little.

The original sentence is the most neutral:

• Os alunos não falam alto durante a prova.

You could also say:

• Durante a prova, os alunos não falam alto.

This puts more focus on during the test.

But you cannot move words around freely the way you might in some other languages. For example, putting não in the wrong place would sound unnatural or incorrect.

A natural approximate pronunciation is:

oz ah-LOO-noos nao FAH-lam AHL-too doo-RAHN-chee ah PROH-vah

A few useful notes:

• os often sounds like oz before a vowel sound
• não has a nasal sound that English does not have exactly
• durante in Brazil is commonly pronounced with chee at the end: doo-RAHN-chee
• stress falls mainly on:
  • aLUnos
  • FAlam
  • duRANte
  • PROva

It could be understood in more than one way, because the Portuguese present tense is flexible.

It may mean:

• a general fact: students do not speak loudly during a test
• a rule: students are not allowed to speak loudly during the test
• a habitual situation: in this class, students do not speak loudly during tests

Without more context, all of these are possible.