Naquela aula, a professora explicou tudo devagar.

Naquela is a contraction of em + aquela.

• em = in / on / at
• aquela = that (feminine singular)

So naquela aula literally means in that class/lesson.

This contraction is very common in Portuguese:

• em + o = no
• em + a = na
• em + os = nos
• em + as = nas
• em + aquele = naquele
• em + aquela = naquela

Because aula is feminine singular, aquela matches it.

Because the sentence needs the idea of in/during that class.

• aquela aula = that class / that lesson
• naquela aula = in that class / during that lesson

In this sentence, the phrase tells you when or in what context the explanation happened, so em is needed, and it contracts to naquela.

In Portuguese, nouns have grammatical gender, and aula is a feminine noun.

That affects the words around it:

• a aula
• aquela aula
• esta aula
• uma aula

This is something you usually just learn with the noun. There is not always a logical reason from meaning alone.

Portuguese uses definite articles much more often than English.

So a professora is completely normal, even when English would just say the teacher or sometimes simply teacher in a different structure.

In Portuguese, professions and subjects often appear with the article:

• A professora explicou...
• O médico chegou cedo.
• A Maria saiu. (in many varieties of Portuguese, including common Brazilian usage)

So here, a professora is the natural way to say the teacher.

Explicou is the preterite of explicar.

• infinitive: explicar = to explain
• preterite: explicou = explained

This tense is used for a completed action in the past.

So the sentence presents the explaining as a finished event:

• Naquela aula, a professora explicou tudo devagar.
• In that class, the teacher explained everything slowly.

This is a very common learner question.

• explicou = a completed action, seen as a whole
• explicava = an ongoing, repeated, habitual, or background action in the past

So:

• A professora explicou tudo devagar.
  = She explained everything slowly on that occasion.

• A professora explicava tudo devagar.
  = She used to explain everything slowly / She was explaining everything slowly.

In your sentence, the speaker is referring to a specific class and a completed event, so explicou fits better.

Because tudo and todo are not used the same way.

• tudo = everything
• todo / toda / todos / todas = all / every, and these usually go with a noun

Examples:

• Ela explicou tudo. = She explained everything.
• Ela explicou toda a lição. = She explained the whole lesson.
• Todos os alunos entenderam. = All the students understood.

So in your sentence, tudo is correct because there is no noun after it. It stands alone and means everything.

Devagar is an adverb here. It means slowly.

It modifies the verb explicou, telling us how the teacher explained everything.

• explicou devagar = explained slowly

Portuguese often uses simple adverbs like this instead of an -mente form.

Yes. Lentamente is also possible, and it also means slowly.

But there is a difference in feel:

• devagar = very common, natural, everyday
• lentamente = more formal, more literary, sometimes a bit less conversational

So:

• A professora explicou tudo devagar. = very natural spoken Portuguese
• A professora explicou tudo lentamente. = correct, but less everyday in tone

Because adverbs of manner often come after the verb or after the object in Portuguese.

Here are some natural possibilities:

• A professora explicou tudo devagar.
• A professora explicou devagar tudo. — possible, but less natural here
• Devagar, a professora explicou tudo. — more marked or stylistic

The version in your sentence is the most neutral and natural one.

Yes, absolutely.

Both are correct:

• Naquela aula, a professora explicou tudo devagar.
• A professora explicou tudo devagar naquela aula.

The difference is mainly focus and rhythm:

• Naquela aula... puts the time/context first
• A professora explicou... starts with the subject

Putting Naquela aula at the beginning helps highlight that specific class.

Because Naquela aula is a fronted time expression.

When a time or place phrase comes first, Portuguese often uses a comma to separate it from the main clause:

• Naquela aula, a professora explicou tudo devagar.
• Ontem, eu cheguei cedo.
• No Brasil, isso é comum.

Sometimes the comma may be omitted in short sentences, but here it is very natural and standard.

A simple pronunciation guide:

• Naquela ≈ nah-KEH-lah
• aula ≈ OW-lah
• a professora ≈ ah pro-feh-SOH-rah
• explicou ≈ es-plee-KOH
• tudo ≈ TOO-doo
• devagar ≈ jeh-vah-GAR or deh-vah-GAR, depending on accent

A few helpful notes for Brazilian Portuguese:

• The x in explicou sounds like s here.
• The final ou in explicou sounds like oh/u in natural speech.
• The d in devagar may sound closer to j before e in many Brazilian accents.
• Stress falls on:
  • naQUEla
  • AUla
  • profesSOra
  • expliCOU
  • TUdo
  • devaGAR

Either can work, depending on context.

• aula can mean class
• aula can also mean lesson

In Naquela aula, English might translate it as:

• In that class
• In that lesson
• During that class

If the context is a school period, class is often the most natural translation. If the focus is on the content being taught, lesson may fit better.

Both.

Portuguese demonstratives can refer to:

• physical distance: that one over there
• time/context already mentioned: that class
• psychological or discourse distance: something less close to the speaker

So naquela aula might refer to:

• a specific class in the past
• a class already mentioned in conversation
• a class seen as more distant from the speaker’s current point of view

In everyday Brazilian Portuguese, demonstratives are often used a bit more flexibly than textbook charts suggest.