Se eu perder o marcador de página, vou esquecer em que página parei de ler.

Because after se when Portuguese talks about a possible future situation, it normally uses the future subjunctive.

So:

• Se eu perder... = If I lose...
• Se eu perco... is not the normal choice for this future meaning

This is very common in Brazilian Portuguese:

• Se eu chegar cedo, te ligo.
• Se chover, a gente fica em casa.
• Se eu perder o marcador de página, vou esquecer...

A useful detail: for many verbs, the future subjunctive looks the same as the infinitive, so perder here is not the infinitive in function, even though it looks the same.

Yes, it can be omitted, because Portuguese verbs usually show the subject clearly.

So both are possible:

• Se eu perder o marcador de página...
• Se perder o marcador de página...

Including eu can make the sentence a little clearer or more emphatic. In everyday Brazilian Portuguese, speakers often include subject pronouns more than in European Portuguese, but omission is still very common.

Both are grammatically correct, but vou esquecer is much more common in everyday spoken Brazilian Portuguese.

• vou esquecer = near-neutral, natural, conversational
• esquecerei = more formal, literary, or less common in normal speech

So:

• vou esquecer em que página parei de ler sounds very natural
• esquecerei em que página parei de ler is correct, but less conversational

This is the same pattern as:

• vou fazer instead of farei
• vou sair instead of sairei

Yes, marcador de página means a physical bookmark, the thing you put inside a book to mark your place.

In Brazil, you may also hear:

• marca-página
• marca-páginas
• sometimes just marcador, if the context is clear

A small nuance:

• bookmark in a browser is usually favorito
• bookmark in a book is marcador de página or marca-página

Because Portuguese often uses definite articles where English may or may not use the.

Here, o marcador de página means the bookmark or my/the bookmark in the situation being discussed. It sounds natural because the speaker has a specific bookmark in mind.

You could also say:

• um marcador de página = a bookmark

That would sound a little less specific.

Because Portuguese needs the preposition em here: literally, in/on what page.

So:

• em que página = on which page / what page

This structure is very common after verbs or expressions that naturally connect with em:

• a página em que parei
• a cidade em que nasci
• o dia em que chegamos

In more informal Brazilian Portuguese, people often say que página or even qual página, but em que página is the more standard and grammatically complete form in this sentence.

In some informal contexts, yes, but it changes the structure a bit.

Natural options include:

• vou esquecer em que página parei de ler — standard and very natural
• vou esquecer qual página eu estava lendo — possible, but it means something slightly different
• vou esquecer em qual página parei de ler — also very common in speech

The original sentence is especially natural because it means I’ll forget what page I stopped reading on.

So em que página is a very good choice here.

Because the two instances do different jobs:

• o marcador de página = bookmark
• em que página parei de ler = what page I stopped reading on

Even though the same word appears twice, it sounds normal because one belongs to the object bookmark, and the other refers to the page number/location in the book.

Portuguese does not avoid this repetition as strongly as English sometimes does.

Because parei de ler clearly means I stopped reading.

• parei by itself = I stopped
• parei de ler = I stopped reading

The de is required after parar when it is followed by another verb:

• parei de estudar
• parei de trabalhar
• parei de ler

So parei de ler is the normal structure.

Because the speaker is referring to the point where they previously stopped reading.

The idea is:

• first, at some earlier moment, I stopped reading
• later, if I lose the bookmark, I will forget where that stopping point was

So parei is in the pretérito perfeito because it refers to a completed action in the past.

Yes, but it would express a slightly different idea.

Compare:

• em que página parei de ler = what page I stopped reading on
• em que página estava lendo = what page I was reading

The original sentence is more precise for the idea of using a bookmark, because a bookmark marks the place where you stopped.

So parei de ler is the best fit here.

Because the sentence begins with a conditional clause:

• Se eu perder o marcador de página, = If I lose the bookmark,
• vou esquecer em que página parei de ler. = main clause

In Portuguese, when a se clause comes first, it is normally separated by a comma.

Compare:

• Se eu tiver tempo, eu vou.
• Se chover, ficamos em casa.

If the main clause came first, the comma is often omitted:

• Vou esquecer em que página parei de ler se eu perder o marcador de página.

Yes. It is an indirect question.

Direct question:

• Em que página parei de ler? = What page did I stop reading on?

Indirect question inside the sentence:

• vou esquecer em que página parei de ler = I’ll forget what page I stopped reading on

Portuguese does this very much like English, except that it does not need do/did support.

Yes, it is natural and correct.

A Brazilian speaker might also say slightly different versions, such as:

• Se eu perder o marcador de página, vou esquecer em que página eu parei de ler.
• Se eu perder o marca-página, vou esquecer onde parei de ler.
• Se eu perder o marcador, vou esquecer em que página parei.

But the original sentence is perfectly normal and clear.

Yes, and that is very common.

• vou esquecer onde parei de ler = I’ll forget where I stopped reading
• vou esquecer em que página parei de ler = I’ll forget what page I stopped reading on

The version with onde is a little less specific and often more conversational. The version with em que página is more explicit because it focuses directly on the page.

Both are good; the original is just more precise.