Um silêncio prolongado pode aumentar a tensão na reunião.

Um is an indefinite article (a / one in English), while o is definite (the).

• Um silêncio prolongado… = A prolonged silence (any such silence, in general)
• O silêncio prolongado… = The prolonged silence (a specific one already known from context)

Here, the idea is general: a long silence in a meeting (not a specific, already mentioned one), so um is more natural.

Yes, it’s grammatically possible, but it sounds more like a headline, a title, or a very general statement (almost like a rule or proverb).

• Um silêncio prolongado pode aumentar… sounds like normal spoken or written Portuguese.
• Silêncio prolongado pode aumentar… is more “headline-like” or stylized.

In everyday sentences, Portuguese usually prefers having an article before singular countable nouns, so um silêncio prolongado is the most natural here.

The default position for most adjectives in Portuguese is after the noun:

• silêncio prolongado = literally silence prolonged

You can say prolongado silêncio, but then the adjective sounds more literary, poetic or emphatic, and sometimes slightly changes the feel:

• um silêncio prolongado – neutral, standard
• um prolongado silêncio – more stylistic, expressive, like something you’d find in literature

For normal, neutral speech, um silêncio prolongado is best.

Both are possible, but there’s a nuance:

• um silêncio prolongado – a prolonged silence (emphasizes that it was extended in time, longer than expected, or unnaturally long)
• um longo silêncio – a long silence (more neutral: it simply lasted a long time)

In many contexts they’re interchangeable, but:

• prolongado often suggests that the duration itself is significant or even uncomfortable.
• longo is more purely descriptive: it just lasted a long time.

All of these are correct, with slightly different tones:

• Um silêncio prolongado pode aumentar a tensão na reunião.
• Um longo silêncio pode aumentar a tensão na reunião.

Pode is from poder (to be able / can / may) and expresses possibility, not certainty.

• Um silêncio prolongado aumenta a tensão… = A prolonged silence increases the tension… (presents it as a general fact)
• Um silêncio prolongado pode aumentar a tensão… = A prolonged silence can increase / may increase the tension… (it happens in some cases)

So pode makes the statement less absolute and more like: “This is something that can happen.”

No. The a in a tensão is the definite article (a = the, feminine singular), not the infinitive marker.

• a tensão = the tension (feminine noun with feminine article)
• aumentar a tensão = to increase the tension

In Portuguese, the infinitive is just the verb form aumentar, without any separate word like English “to” in front.

Portuguese tends to use definite articles more than English does, especially with abstract nouns:

• English: increase tension
• Portuguese: aumentar a tensão

Here, a tensão refers to the tension specific to that meeting. Without the article, aumentar tensão sounds incomplete or unidiomatic in Portuguese in this context. So a tensão is the normal choice.

Na is a contraction:

• em (in / at) + a (the, feminine) → na

So:

• em a reunião → na reunião = in the meeting / at the meeting

Portuguese always contracts these when spoken and written:

• em + a → na
• em + o → no
• em + as → nas
• em + os → nos

Yes, but it changes the meaning and usage a bit.

• na reunião = in the meeting / at the meeting (a specific meeting)
• em reunião = literally in meeting, used more like “in a meeting” as a state or activity, often without specifying which meeting.

Examples:

• Estou na reunião. = I am in the (specific) meeting.
• Estou em reunião. = I am in a meeting (busy in a meeting, not available).

In your sentence, you’re talking about a specific meeting situation, so na reunião is the natural choice.

In Portuguese, grammatical gender is largely arbitrary and must be memorized.

• reunião ends with -ão, and many -ão nouns are masculine (o avião, o coração), but some are feminine (a reunião, a estação).

There isn’t a reliable, simple rule here; reunião is just one of those nouns you have to learn as feminine:

• a reunião
• uma reunião
• na reunião

Yes, that’s perfectly correct and very natural.

• Na reunião, um silêncio prolongado pode aumentar a tensão.

Putting na reunião at the beginning simply emphasizes the context (“in the meeting”) more. The basic meaning is the same; it’s a matter of focus and style, not grammar correctness.

Place não directly before the verb pode:

• Um silêncio prolongado não pode aumentar a tensão na reunião.
  = A prolonged silence cannot increase the tension in the meeting.

Pattern:
[subject] + não + [conjugated verb] + [rest of the sentence]

You need to pluralize the article, noun, adjective, and verb:

• Uns silêncios prolongados podem aumentar a tensão na reunião.

Changes:

• um → uns (some / a few)
• silêncio → silêncios
• prolongado → prolongados (agrees with silêncios)
• pode → podem (agrees with silêncios)

A tensão and na reunião stay the same because they are still singular.