Они спокойно говорили о разводе.

Because the subject is они = they.

In Russian past tense, the verb agrees with the subject in number, and in the singular it also agrees in gender:

• он говорил = he was talking / talked
• она говорила = she was talking / talked
• оно говорило = it was talking / talked
• они говорили = they were talking / talked

So говорили is simply the correct past-tense plural form for они.

Говорят is present tense: they speak / they are speaking.

This sentence is in the past, so Russian uses говорили:

• они говорят = they are speaking / they speak
• они говорили = they were speaking / they talked

So говорили tells you the action happened in the past.

Because говорить and сказать are not used in exactly the same way.

• говорить = to speak, to talk, to say in a general or ongoing sense
• сказать = to say, usually one completed act of saying something

In Они спокойно говорили о разводе, the idea is that they were talking/discussing the topic, not just producing one finished statement.

So:

• говорили о разводе = were talking about the divorce / talked about the divorce
• сказали о разводе would sound more like said something about the divorce

Because the preposition о means about, and it requires the prepositional case.

The noun развод changes form after о:

• dictionary form: развод
• after о: о разводе

So:

• говорить о чём? = to talk about what?
• о разводе = about the divorce / about divorce

This is a very common pattern:

• о книге = about the book
• о работе = about work
• о семье = about the family

It is the prepositional singular form of развод.

The base noun is:

• развод = divorce

After о, it becomes:

• о разводе = about the divorce

So the sentence contains:

• они — subject
• спокойно — adverb
• говорили — verb
• о разводе — prepositional phrase

It can mean either, depending on context.

Russian imperfective past often corresponds to both:

• were talking when you want to stress the process or ongoing action
• talked when English uses a simple past but the Russian still presents it as an activity, not a completed result

So говорили does not force a single English translation. The surrounding context decides what sounds best.

Спокойно means calmly, peacefully, or without agitation.

So the sentence suggests that they discussed the divorce in a calm way, without arguing or getting emotional.

It modifies the verb говорили:

• говорили = talked
• спокойно говорили = talked calmly

Yes. Russian word order is fairly flexible.

The neutral order here is:

• Они спокойно говорили о разводе.

But you could also see:

• О разводе они говорили спокойно.
• Спокойно они говорили о разводе.

These versions are grammatically possible, but they shift the emphasis a little:

• О разводе first emphasizes the topic
• спокойно first emphasizes the calm manner

The original sentence is the most neutral and natural if you are just stating the fact.

Yes, Russian can sometimes omit the subject pronoun when it is clear from context.

So in context, you might hear:

• Спокойно говорили о разводе.

That could mean They were calmly talking about the divorce, if the listener already knows who they are.

But including они makes the subject explicit. That is often done when:

• introducing the sentence
• making the subject clear
• contrasting they with someone else

It could mean either one, because Russian has no articles like a and the.

So о разводе can mean:

• about divorce in a general sense
• about the divorce if a specific divorce is understood from context

English has to choose, but Russian often leaves that to context.

A natural stress pattern is:

Они́ споко́йно говори́ли о разво́де.

Word stress:

• они́
• споко́йно
• говори́ли
• разво́де

A rough pronunciation guide:

• они ≈ a-NEE
• спокойно ≈ spa-KOY-na
• говорили ≈ ga-va-REE-li
• о разводе ≈ a raz-VO-dye

As often happens in Russian, unstressed о is pronounced more like a.

Yes, and it would also be natural:

• Они спокойно разговаривали о разводе.

Both can mean They were calmly talking about the divorce.

A small difference in feel:

• говорили can mean spoke / said / talked
• разговаривали more specifically means were conversing / talking with each other

In this sentence, both work, but говорили о разводе is very common and natural.