A reunião online está prestes a começar.

A is the feminine singular definite article, meaning the.

You often use the article in Portuguese where English also uses the:

• a reunião = the meeting

Since reunião is a feminine noun, it takes a, not o.

You learn the gender as part of the noun: a reunião.

There is not always a perfect rule, but many nouns ending in -ão can be masculine or feminine, so you should memorize them with the article. In this case:

• a reunião = the meeting

That also affects other words in the sentence, such as:

• a reunião
• a reunião online
• the phrase uses está, which agrees with the subject, not with gender

Because estar prestes a is a fixed expression meaning to be about to.

Portuguese often uses estar for temporary states, ongoing situations, or expressions like this one. So:

• está prestes a começar = is about to start

You would not normally say é prestes a here.

Prestes a means about to, on the verge of, or just about ready to do something.

So:

• está prestes a começar = is about to begin

It is a very common and natural expression in both spoken and written Portuguese.

More examples:

• Estou prestes a sair. = I’m about to leave.
• Ela está prestes a ligar. = She is about to call.

In the expression estar prestes a + infinitive, the a is required.

So the pattern is:

• estar prestes a fazer algo
• estar prestes a começar
• estar prestes a sair

That a is not the feminine article here. It is part of the verbal expression.

Because after estar prestes a, Portuguese uses the infinitive form of the next verb.

So:

• está prestes a começar = is about to start
• está prestes a terminar = is about to end
• está prestes a acontecer = is about to happen

This is very similar to English to be about to + verb.

In Portuguese, adjectives often come after the noun, unlike in English.

So:

• reunião online = online meeting
• literally: meeting online

This is very normal word order in Portuguese.

That said, online here functions more like a descriptive modifier than a classic adjective, but in practice learners can think of it as following the usual Portuguese pattern: noun first, description after.

Yes, online is widely used and sounds natural in Brazilian Portuguese.

Common alternatives include:

• reunião online
• reunião virtual

Both are natural. Reunião online is extremely common in everyday use.

You may also see spelling variations such as:

• online
• on-line

But online is very common in modern Brazilian usage.

Yes. Começar can translate as both to start and to begin, depending on context.

Examples:

• A aula começou. = The class started / began.
• Vamos começar. = Let’s start / begin.

So in this sentence, either start or begin is a good translation.

Yes, but the meaning changes slightly.

• A reunião online vai começar. = The online meeting is going to start.
• A reunião online está prestes a começar. = The online meeting is about to start.

Está prestes a começar suggests it will happen very soon, almost immediately.
Vai começar is more neutral and simply indicates a future action.

Yes, it is completely natural.

It sounds a little more polished than very casual speech, but it is still common and normal. You might hear it in:

• workplace announcements
• event platforms
• online classes
• automated messages

A slightly simpler everyday version would be:

• A reunião online vai começar.

But the original sentence is perfectly natural.

A rough pronunciation guide is:

• reunião ≈ heh-oo-nee-OWN in many Brazilian accents

A few useful notes:

• the r at the beginning often sounds like an English h in Brazilian Portuguese
• the ending -ão is nasal, which is important
• the stress is on the last syllable: reu-ni-ÃO

So the nasal ending is one of the trickiest parts for English speakers.

Yes.

• A reunião está prestes a começar. = The meeting is about to start.

That is also fully correct. Online just adds extra information about what kind of meeting it is.

Yes, that is also possible, but it is a bit different in tone.

• está prestes a começar = is about to start, very close to happening
• está para começar = is about to start / is due to start

Both are correct, but prestes a often feels a little more explicit about the action being imminent.

Because the subject is singular:

• A reunião online = the online meeting

Since there is only one meeting, the verb is singular:

• A reunião online está...

If it were plural, you would say:

• As reuniões online estão prestes a começar. = The online meetings are about to start.