Às vezes, um casal decide se separar depois de muito tempo.

Às vezes is a fixed expression meaning sometimes.

A few useful details:

• vez = time/occasion
• vezes = times/occasions
• às = contraction of a + as

Historically, the phrase comes from the idea of at the times / on some occasions, and in modern Portuguese you simply learn às vezes as one chunk meaning sometimes.

Important: the accent in às is not the same as the accent in words like avó. It marks a contraction: a + as = às.

Because Às vezes is an adverbial expression placed at the beginning of the sentence, and Portuguese often separates that kind of introductory element with a comma.

So this is very natural:

Às vezes, um casal decide se separar...

You may also see short introductory expressions without a comma in informal writing, but here the comma is standard and helps readability.

Because casal is grammatically a singular noun in Portuguese, even though it refers to a pair.

So:

• um casal = a couple
• dois casais = two couples

This is similar to English, where a couple is also singular in form even though it refers to two people.

Usually, yes. Casal normally means a romantic pair, such as spouses or partners.

It can refer to:

• a married couple
• a dating couple
• a long-term romantic pair

If you just want to say two people without the romantic idea, you would not use casal.

In se separar, the se is part of the verb expression se separar, which means to separate / to break up / to split up when talking about people in a relationship.

Here, se is a reflexive/pronominal element. In many cases, Portuguese uses these pronouns where English does not.

So:

• separar can mean to separate something
• se separar means to separate oneself / to break up

Examples:

• Eles vão separar os livros. = They are going to separate the books.
• Eles vão se separar. = They are going to separate / break up.

Not with the same meaning.

• decide se separar = decides to break up / separate
• decide separar would suggest decides to separate something, and it would usually need an object

For example:

• Ela decide separar as roupas. = She decides to separate the clothes.

So in your sentence, se is necessary because the idea is that the couple separates from each other.

Because after another verb like decide, Portuguese often uses the infinitive with the pronoun attached to it or placed before it, depending on style and structure.

Here, decide se separar is the normal, natural wording.

You are attaching the pronoun to the infinitive idea, not to decide itself.

Compare:

• decide se separar = decides to separate
• quer se mudar = wants to move
• precisa se cuidar = needs to take care of oneself

In Brazilian Portuguese, placing se before the infinitive like this is very common and natural.

Because it follows decide.

In Portuguese, just as in English, verbs like decide, want, need, prefer are commonly followed by another verb in the infinitive:

• decide sair = decides to leave
• quer viajar = wants to travel
• precisa estudar = needs to study

So:

• decide = the conjugated verb
• se separar = the infinitive expression that completes the idea

Decide is third person singular present tense of decidir.

• eu decido
• você/ele/ela decide

In this sentence, the present tense is being used for a general truth / common occurrence:

Às vezes, um casal decide se separar...
= Sometimes, a couple decides to separate...

This is very common in both Portuguese and English when talking about things that happen in general.

Because depois normally takes the preposition de before a noun phrase.

So:

• depois de muito tempo = after a long time
• depois de anos = after years
• depois do trabalho = after work

Without de, the phrase would be incorrect here.

Because muito tempo here is a general expression meaning a long time, not a specific identified time.

So:

• depois de muito tempo = after a long time

You use do when you mean de + o, with a specific masculine singular noun:

• depois do almoço = after lunch
• depois do filme = after the movie

But muito tempo is not introduced by o here, so you use just de.

Muito tempo means a lot of time / a long time.

Portuguese often uses tempo in the singular in this kind of expression, just as English often says a lot of time rather than many times.

Compare:

• muito tempo = a long time / much time
• muitas vezes = many times

So:

• depois de muito tempo = after a long time
• depois de muitas vezes would mean something completely different

Not naturally in this context.

• casal = couple, especially romantic
• par = pair, matching set, or two items together

Examples:

• um casal feliz = a happy couple
• um par de sapatos = a pair of shoes

So for a relationship, casal is the correct word.

The original sentence is already natural. In speech, the main difference would be rhythm and pronunciation, not grammar.

A speaker might say it smoothly like:

Às vezes, um casal decide se separar depois de muito tempo.

A slightly more conversational alternative could be:

Às vezes, um casal resolve se separar depois de muito tempo.

Here, resolve can mean something like ends up deciding or makes up their mind to. But your original sentence is perfectly good and natural.

It is neutral.

It works well in:

• conversation
• writing
• articles
• explanations
• everyday speech

Nothing in it is especially slangy or especially formal. That makes it a very useful model sentence for learners.