O assunto ficou mais claro depois que a professora explicou de novo.

O assunto usually means the subject, the topic, or the matter.

In this sentence, the most natural translation is the topic or the subject matter, not school subject like math or history.

So here o assunto is the thing being discussed or explained.

Ficou comes from ficar, which can mean different things depending on context.

Here, ficou mais claro means became clearer or got clearer.

So although ficar can sometimes mean to stay, in this sentence it means a change of state:

• ficou mais claro = became clearer

This is very common in Portuguese:

• ficou difícil = it became difficult
• ficou fácil = it became easy
• ficou melhor = it got better

Because the sentence is talking about a change, not just a state.

• estava mais claro = it was clearer
  This describes a condition.
• ficou mais claro = it became clearer / it got clearer
  This shows that something changed after the explanation.

So ficou is the best choice because the topic was not clear enough before, and then it became clearer.

Not here. Claro can mean light/bright in a literal sense, but it also means clear in the sense of easy to understand.

So:

• uma sala clara = a bright room
• uma explicação clara = a clear explanation

In o assunto ficou mais claro, mais claro means easier to understand.

Because claro agrees with o assunto, and assunto is a masculine noun.

• o assunto → masculine singular
• so the adjective is claro

If the noun were feminine, it would change:

• a explicação ficou mais clara

Depois que means after in the sense of after something happened.

In this sentence:

• depois que a professora explicou de novo = after the teacher explained again

It introduces a clause with a verb.

Compare:

• depois que ela explicou = after she explained
• depois da aula = after the class
• depois de estudar = after studying

Because depois que is the normal expression before a clause with a conjugated verb.

Use:

• depois que + clause
  • depois que a professora explicou

Use:

• depois de + noun
  • depois da explicação
• depois de + infinitive
  • depois de explicar

Depois de que is generally not the natural choice here.

Explicou is the preterite of explicar, and it is used because the teacher’s explanation is seen as a completed action.

• a professora explicou de novo = the teacher explained again

That explanation happened, was completed, and then as a result the topic became clearer.

This is why the preterite works well in both parts of the sentence:

• ficou = became
• explicou = explained

Yes, here de novo means again.

So:

• explicou de novo = explained again

It is very common in everyday Brazilian Portuguese.

Other possibilities are:

• novamente = again, more formal
• outra vez = again

Examples:

• Pode repetir de novo? = Can you repeat again?
• Ela explicou novamente. = She explained again.

In everyday speech, de novo is extremely common.

Yes.

• novo = new
  • um livro novo = a new book
• de novo = again
  • explicar de novo = explain again

So even though they contain the same word, the meaning is different because de novo is a fixed expression.

Portuguese often uses definite articles more broadly than English does.

So a professora is perfectly natural and often preferred when talking about a specific teacher already understood from context.

Brazilian Portuguese commonly uses articles before professions or roles when referring to a known person:

• A professora explicou
• O médico chegou
• A diretora falou com os alunos

Leaving out the article is sometimes possible in special contexts, but a professora is the normal choice here.

Yes, but the original order is very natural.

Original:

• O assunto ficou mais claro depois que a professora explicou de novo.

You could also say:

• Depois que a professora explicou de novo, o assunto ficou mais claro.

Both are correct. The difference is mainly in emphasis:

• original order emphasizes the result
• fronting depois que... emphasizes the time sequence

Yes. A few natural alternatives are:

• ficou mais fácil de entender = became easier to understand
• ficou mais compreensível = became more understandable
• ficou mais entendido = not natural here

The original ficou mais claro is one of the most natural and common ways to express this idea.

It is neutral and very natural. It works well in both speech and writing.

Some parts can shift the register slightly:

• de novo sounds more everyday and conversational
• novamente would sound a bit more formal

So:

• A professora explicou de novo = natural, everyday
• A professora explicou novamente = a little more formal

But the full sentence is standard Portuguese either way.