Depois da reunião, eu vou explicar tudo com mais calma.

Because da is the contraction of de + a.

• de = of / from / after in many contexts
• a = the feminine singular definite article the

So:

• de + a reunião → da reunião

This kind of contraction is very common in Portuguese:

• de + o → do
• de + a → da
• de + os → dos
• de + as → das

Since reunião is a feminine noun, it takes a, so da reunião is the correct form.

When depois is followed by a noun or an action, it normally takes de.

Examples:

• depois da reunião = after the meeting
• depois do almoço = after lunch
• depois de falar com ela = after speaking with her

But depois can also appear by itself when the rest is understood:

• Eu faço isso depois. = I’ll do that later.

So in this sentence, depois da reunião is the normal way to say after the meeting.

Because Depois da reunião is an introductory time expression at the beginning of the sentence.

In Portuguese, a comma is often used after opening phrases like:

• Depois da reunião, ...
• Amanhã, ...
• Na semana que vem, ...

It helps separate the time setting from the main clause. In short:

• Depois da reunião, eu vou explicar... sounds natural and well punctuated.
• Without the comma, people would still understand it, but the comma is standard and clearer.

Yes, eu can be omitted.

Portuguese often drops subject pronouns when the verb already makes the subject clear:

• (Eu) vou explicar tudo com mais calma.

Both are correct.

Why include eu?

• for emphasis
• for clarity
• for contrast with someone else

Examples:

• Eu vou explicar, não ele. = I’ll explain it, not him.
• Vou explicar tudo com mais calma. = perfectly natural in everyday speech

So eu is optional here, but not wrong at all.

Vou explicar is the very common spoken way to express the future in Brazilian Portuguese.

It is formed with:

• ir in the present tense + infinitive

So:

• vou explicar = I am going to explain / I will explain

The simple future explicarei is grammatically correct, but it sounds:

• more formal
• more written
• less common in everyday conversation

In Brazil, people usually prefer:

• eu vou explicar rather than
• eu explicarei

Both mean the future, but vou explicar is much more natural in normal speech.

Com mais calma is a very common expression. It usually means something like:

• more calmly
• when things are calmer
• more carefully
• in a less rushed way
• in more detail, depending on context

It does not just mean literal emotional calmness. Often it suggests:

• there isn’t enough time now
• now is too hectic
• later the speaker can explain better and more thoroughly

So in this sentence, vou explicar tudo com mais calma often means:

• I’ll explain everything later in a calmer, clearer, less rushed way.

Both are possible, but com mais calma is much more natural and common in everyday Brazilian Portuguese.

Compare:

• Vou explicar tudo com mais calma. = very natural
• Vou explicar tudo mais calmamente. = understandable, but less common and a bit more formal or bookish

Portuguese often prefers noun-based expressions like com calma or com mais calma instead of the adverb ending in -mente.

Other examples:

• Fala com calma. = Speak calmly.
• Vamos resolver isso com calma. = Let’s deal with this calmly.

So com mais calma is the idiomatic choice.

Here mais means more, but the comparison is often only implied.

Com mais calma usually means:

• more calmly than now
• with more time than we have now
• in a less rushed way than the current situation allows

Portuguese often leaves that comparison unstated because it is obvious from context.

So the idea is:

• not now, while things are busy
• later, with more calm / time / patience

Because tudo is the direct object of explicar.

• explicar = to explain
• tudo = everything

So:

• explicar tudo = to explain everything

This is the normal word order in Portuguese:

• verb + object

Examples:

• vou fazer isso = I’m going to do that
• quero entender tudo = I want to understand everything
• preciso explicar o problema = I need to explain the problem

So vou explicar tudo is completely standard.

Yes. If you want to specify the person receiving the explanation, you can add an indirect object.

Examples:

• Depois da reunião, eu vou explicar tudo para você com mais calma.
• Depois da reunião, vou te explicar tudo com mais calma.

Both are common in Brazil, but they are a little different in style:

• para você = more explicit, very clear
• te = shorter, very common in speech

Without any of that, vou explicar tudo simply means I’ll explain everything, and the listener is often understood from context.

Yes, reunião is feminine:

• a reunião
• da reunião

You usually learn noun gender together with the article:

• a reunião = the meeting

The ending -ão does not automatically tell you the gender. Some -ão nouns are masculine, some are feminine.

Examples:

• o coração = the heart
• o avião = the airplane
• a reunião = the meeting
• a situação = the situation

So the safest habit is to memorize the noun together with o or a.

Reunião has stress on the last syllable: reu-ni-ÃO.

A few pronunciation points:

• ão is a nasal sound, very common in Portuguese.
• It is not pronounced like English ow.
• The final sound is nasal, something like ow/ng said through the nose, but not exactly either English sound.

A rough English-friendly guide:

• reunião ≈ heh-oo-nee-OW̃ But that is only approximate.

Also:

• the ião part forms one flowing ending
• the tilde-like nasal quality is important

If you want to sound more natural, focus especially on that nasal -ão ending, since it appears in many common words:

• não
• então
• situação
• informação

Yes. Após a reunião is correct and means basically the same thing.

Compare:

• Depois da reunião = very common, neutral, everyday
• Após a reunião = also correct, but often a bit more formal or written

In spoken Brazilian Portuguese, depois da reunião is usually the more natural choice.

Yes, absolutely. That would sound very natural.

• Depois da reunião, eu vou explicar tudo com mais calma.
• Depois da reunião, vou explicar tudo com mais calma.

Both are correct. The version without eu is often more natural in conversation unless the speaker wants emphasis.

Portuguese often leaves out subject pronouns because the verb form already shows the person:

• vou clearly points to eu

So dropping eu is normal, especially in Brazilian speech.