O último capítulo ficou por comentar na aula.

It uses the very common Portuguese structure ficar por + infinitive.

In this sentence, ficou por comentar means something like:

• was left uncommented on
• remained to be discussed
• didn’t end up being discussed

So this is not just the literal idea of ficar = to stay. The full pattern ficar por fazer often means to be left undone.

Some similar examples:

• Ficou por fazer. = It was left undone.
• Muita coisa ficou por dizer. = A lot was left unsaid.
• O relatório ficou por acabar. = The report was left unfinished.

So here, O último capítulo ficou por comentar na aula means the last chapter was not discussed in class and remained pending.

Because Portuguese is not using a normal passive here. It is using the idiomatic construction ficar por + infinitive.

Compare:

• O último capítulo ficou por comentar.
  = The last chapter was left to be discussed / remained undiscussed.

• O último capítulo foi comentado.
  = The last chapter was commented on / discussed.

• O último capítulo não foi comentado.
  = The last chapter was not discussed.

With ficou por comentar, the idea is not just that the action did not happen. It also suggests that it was still pending or left unfinished.

So ficou adds the idea of a resulting state: after the class, that chapter was still left to discuss.

Because in this expression, Portuguese specifically uses ficar por + infinitive.

That is a fixed and very common structure meaning:

• to remain to be done
• to be left to do
• to be left undone

So:

• ficar por comentar = to remain to be discussed
• ficar por fazer = to remain to be done

By contrast, para comentar usually means in order to comment or for commenting on, depending on context. It does not normally create the same idiomatic meaning.

So:

• ficou por comentar = correct for was left undiscussed
• ficou para comentar = would suggest something more like was left for later discussion, which is a bit different and more specific

Both can exist, but they do not mean exactly the same thing.

Because the sentence is describing the result of a completed situation in the past — presumably after the lesson ended.

Ficou is the pretérito perfeito of ficar. In European Portuguese, this tense is often used for a completed event or change of state.

Here the idea is:

• during the lesson, they did not get to that chapter
• by the end, it ended up still not discussed

So ficou works well because it expresses the outcome:

• it ended up left undiscussed
• it remained undiscussed

If you changed the tense, the meaning would shift:

• fica por comentar = is left to discuss / remains to be discussed
• ficava por comentar = used to be left to discuss / was being left to discuss, depending on context
• ficou por comentar = was left to discuss / ended up not being discussed

Portuguese normally uses the definite article much more often than English.

So O último capítulo is the natural standard form for the last chapter.

You can sometimes drop the article in headlines, notes, titles, or very compressed styles, for example:

• Último capítulo por comentar

But in a normal full sentence, O último capítulo is what you would expect.

This is one of the big differences from English: Portuguese often prefers an article where English might not emphasize it as much.

Because adjective position in Portuguese is flexible, but not random.

In this case, último normally goes before the noun:

• o último capítulo = the last chapter

That is the standard, natural order here.

If you put the adjective after the noun, it can sound unusual, literary, or it may create a different nuance depending on the adjective. For último, the pre-nominal position is the one learners should use in this meaning.

So for ordinary Portuguese:

• o último capítulo = natural
• o capítulo último = unnatural in everyday language

Here comentar means something like:

• to comment on
• to discuss
• to talk about
• to go over

In a classroom context, comentar often means discussing a text, chapter, topic, or idea aloud.

So although English comment can sound a bit narrower, Portuguese comentar is often quite natural in educational contexts:

• comentámos o texto na aula = we discussed/commented on the text in class

So in this sentence, it does not mean just making a quick remark. It can imply a proper class discussion or explanation.

Because Portuguese contracts em + a into na.

So:

• em + a = na
• em + o = no
• em + as = nas
• em + os = nos

That means:

• na aula = in the class / in class / during the lesson

This contraction is required in normal Portuguese.

So you would say:

• na aula
• not em a aula

Usually during the lesson or in class, not necessarily physically inside the classroom.

So:

• na aula often means during class
• na sala de aula is more specifically in the classroom

In your sentence, na aula most naturally means that during that lesson, the last chapter was not discussed.

Yes. That is perfectly correct, but it is slightly different in nuance.

• O último capítulo não foi comentado na aula.
  = The last chapter was not discussed in class.

This simply states that the discussion did not happen.

• O último capítulo ficou por comentar na aula.
  = The last chapter was left undiscussed in class / remained to be discussed.

This version often suggests more clearly that it was left pending or unfinished.

So both are correct, but ficou por comentar is more idiomatic if you want the idea of something that remained still to do.

Not normally in this exact meaning.

The idiomatic structure here is ficar por + infinitive.

• ficou por comentar = natural
• ficou sem comentar = not the usual choice here

You might hear related structures with sem, but they are built differently, for example:

• ficou sem ser comentado = it remained without being discussed / it went undiscussed

That is possible, but it is heavier and less elegant than ficou por comentar.

So the best version for a learner to remember is:

• ficar por comentar
• ficar por fazer
• ficar por dizer

Not exactly in the standard grammatical sense.

A true passive would be:

• O último capítulo não foi comentado na aula.

Your sentence uses the active verb ficar in an idiomatic construction:

• ficou por comentar

Even though it translates into English somewhat like a passive idea, grammatically it is better to think of it as a set expression meaning to be left to do / to remain undone.

So it has a passive-like meaning, but the structure itself is not the normal passive with ser + past participle.

The subject is O último capítulo.

That is the thing that ficou por comentar.

So the structure is:

• O último capítulo = subject
• ficou = verb
• por comentar = complement that completes the idiomatic meaning
• na aula = adverbial phrase of context/place/time

A useful way to see it is:

• [The last chapter] [was left undiscussed] [in class].

Yes — very often. This is a highly useful pattern in Portuguese.

Some common examples:

• Ficou por fazer. = It was left undone.
• Ficou por dizer. = It was left unsaid.
• Ficou por resolver. = It remained unresolved.
• Ficou por explicar. = It was left unexplained.
• Ficou por ler. = It was left unread / remained to be read.

So this sentence is a very good model for a productive structure:

ficar por + infinitive = to remain left to do / to be left undone