Eu deixei espaço na margem porque queria escrever outro comentário depois da aula.

Yes. In Brazilian Portuguese, Eu is often optional because the verb form already shows the subject.

• Eu deixei espaço... = I left space...
• Deixei espaço... also means I left space...

Here, deixei clearly tells us the subject is I.
Speakers may include Eu for:

• emphasis,
• contrast,
• clarity.

So both are natural:

• Eu deixei espaço na margem...
• Deixei espaço na margem...

Deixei is the preterite form of deixar.

• deixar = to leave, to let
• eu deixei = I left

The preterite is used for a completed action in the past. In this sentence, the speaker is talking about one finished action: they left some space.

Compare:

• Eu deixei espaço. = I left space.
• Eu deixava espaço. = I used to leave space / I was leaving space.

Yes, here deixar means to leave in the sense of leaving something available or unused.

So deixei espaço means:

• I left space
• I left some room

It does not mean to leave a place here. The context makes that clear.

Some other uses of deixar:

• Deixei o livro na mesa. = I left the book on the table.
• Deixa eu falar. = Let me speak.
• Ele deixou o emprego. = He left the job.

Both are possible, but they feel slightly different.

• deixei espaço = I left space / I left some space
• deixei um espaço = I left a space / one space / a gap

Without the article, Portuguese often expresses a more general or uncounted idea, similar to English space.
With um, it can sound more specific, as if you mean one particular blank area or gap.

In this sentence, deixei espaço na margem sounds very natural because the idea is simply that there was room left in the margin.

Na is a contraction of:

• em
  • a = na

So:

• na margem = in the margin / on the margin

This is extremely common in Portuguese.

Other examples:

• no livro = em + o
• na aula = em + a
• nos cadernos = em + os

Even though English says in the margin or on the margin depending on context, Portuguese naturally uses na margem here.

No. Margem can have several meanings depending on context:

• the margin of a page
• the bank/edge of a river
• figurative margin, as in room for error or profit margin

In this sentence, because of escrever outro comentário, it clearly means the margin of a page or notebook.

Here porque means because, so it is written as one word.

• porque = because

Compare:

• Eu saí porque estava cansado. = I left because I was tired.

Por que as two words is usually used in questions or in structures meaning for what reason:

• Por que você saiu? = Why did you leave?

So in your sentence, porque is correct because it introduces the reason.

This is a very common learner question.

• quis = preterite of querer
• queria = imperfect of querer

Here queria is used because it expresses an ongoing mental state, intention, or background reason in the past:

• porque queria escrever... = because I wanted to write...

This sounds more natural than quis in this context.

Why?

The sentence has:

• a completed action: deixei
• a background motivation: queria

That combination is very common in Portuguese:

• Saí cedo porque estava cansado.
• Fechei a janela porque fazia frio.
• Deixei espaço porque queria escrever mais tarde.

If you say porque quis escrever, it sounds more like because I decided to write or because I did want to write, which is less natural here.

In most cases here, you should understand it as wanted to write.

The imperfect tense in Portuguese often covers ideas that English can express in different ways:

• wanted
• used to want
• was wanting

But in this sentence, the natural English meaning is simply:

• because I wanted to write another comment later

So even though queria is imperfect, you usually do not translate it literally as was wanting.

Outro means another or other.

So outro comentário means:

• another comment
• one more comment

It implies there was already a comment, or at least that the speaker expected to write an additional one.

Examples:

• Quero outro café. = I want another coffee.
• Ela comprou outro livro. = She bought another book.

In Portuguese, outro normally comes before the noun:

• outro comentário
• outra ideia
• outros alunos

This is the normal word order.
Saying comentário outro would sound unnatural in standard usage.

Because de + a usually contracts to da.

So:

• depois da aula = after the class

This is another very common contraction in Portuguese:

• do = de + o
• da = de + a
• dos = de + os
• das = de + as

So depois da aula is the normal form.

It can mean any of those depending on context.

Common meanings of aula:

• class
• lesson
• lecture
• session

In this sentence, depois da aula most naturally means after class or after the lesson.

Yes, you could say após a aula, and it would mean the same thing: after class.

However:

• depois da aula is more common in everyday speech
• após a aula can sound a bit more formal or written

So for normal conversation, depois da aula is usually the best choice.

Yes, depois da aula can move, depending on what you want to emphasize.

Original:

• ...porque queria escrever outro comentário depois da aula.

This most naturally means the writing would happen after class.

You could also say:

• ...porque depois da aula queria escrever outro comentário.

That gives a little more emphasis to after class.

In general, Portuguese word order is flexible, but the original sentence is very natural and clear.

The original sentence is already natural. In speech, a Brazilian speaker might also say:

• Deixei espaço na margem porque queria escrever outro comentário depois da aula.
• Eu deixei um espaço na margem porque queria escrever mais um comentário depois da aula.

Possible small variations:

• dropping Eu
• using um espaço
• using mais um comentário instead of outro comentário

All of these are natural, but your original sentence is perfectly good Brazilian Portuguese.