Eu uso um marcador de página para não perder a página em que parei de ler.

Yes. In Brazilian Portuguese, the subject pronoun is often optional, especially with verbs whose form already shows who the subject is.

So:

Eu uso um marcador de página...
and
Uso um marcador de página...

both work.

Including Eu can make the sentence clearer, more emphatic, or more natural in some contexts. In everyday Brazilian Portuguese, subject pronouns are used more often than in European Portuguese, so Eu uso sounds completely normal.

Uso is the first-person singular present tense of usar: to use.

So eu uso = I use.

Examples:

• Eu uso óculos. = I wear glasses.
• Eu uso esse aplicativo todo dia. = I use that app every day.

Marcador by itself means marker or something used to mark. Adding de página specifies what kind of marker it is: a bookmark.

So:

• marcador = marker / highlighter / bookmark, depending on context
• marcador de página = bookmark

In real usage, Brazilians may also say:

• marcador de livro
• marca-página / marca-páginas

But marcador de página is perfectly understandable.

Because in Portuguese, this kind of noun + de + noun structure often uses the second noun in the singular to express the general category.

So marcador de página literally means something like page marker, not marker of one specific page.

Similar patterns:

• escova de dente = toothbrush
• copo de vinho = wine glass
• máquina de lavar = washing machine

You may also hear plural forms like marcador de páginas, but the singular version is very common in these compound-like expressions.

Para means in order to / so as to / for, and não perder means not to lose.

So para não perder... means:

• so as not to lose...
• in order not to lose...

This is a very common structure in Portuguese:

para + infinitive
para não + infinitive

Examples:

• Estudo para aprender. = I study to learn.
• Saí cedo para não me atrasar. = I left early so I wouldn’t be late.

Because não normally goes before the verb it negates.

Here, the verb is the infinitive perder (to lose), so:

• não perder = not to lose

This is the normal order in Portuguese.

Compare:

• Quero dormir. = I want to sleep.
• Quero não sair. is usually unnatural in most normal situations.
• Quero não perder o começo. = I want not to miss the beginning.
  This is possible, but usually sounds more formal or marked.

In your sentence, para não perder is the natural standard structure.

Literally, yes: to lose the page. But in this context it means to lose your place in the book, meaning you forget where you stopped reading.

In English, we would more naturally say:

• so I don’t lose my place
• so I can remember where I stopped

Portuguese often expresses this as perder a página or perder a página em que parei.

Another natural alternative would be:

• para não perder o lugar
• para não perder a página onde parei

Because the first página belongs to the expression marcador de página (bookmark), while the second a página refers to the specific page where the person stopped reading.

So the structure is:

• um marcador de página = a bookmark
• para não perder a página... = so as not to lose the page...

Even though English might avoid repeating page, Portuguese often allows this kind of repetition if it is clear and natural.

Em que means on which / where here.

The verb or idea requires the preposition em because you stop on a page.

So:

• a página em que parei de ler
  = the page on which I stopped reading = more naturally in English, the page where I stopped reading

This is a very common structure in Portuguese:

• a cidade em que nasci = the city where I was born
• o dia em que cheguei = the day when I arrived
• o assunto em que pensei = the topic I thought about

Yes, many Brazilians would naturally say:

a página onde parei de ler

In everyday Brazilian Portuguese, onde is often used where careful grammar would prefer em que.

So:

• a página em que parei de ler = more formally precise
• a página onde parei de ler = very common in speech

If you want a safe, standard choice for writing, em que is excellent.

Because the verb parar normally takes the preposition de before another verb in the infinitive when it means to stop doing something.

So:

• parei de ler = I stopped reading
• parou de falar = he/she stopped talking
• paramos de trabalhar = we stopped working

Without de, it would sound wrong in standard Portuguese in this meaning.

No. Parei de ler means I stopped reading at some point. It does not mean you finished the book.

It usually implies you paused and may continue later.

If you wanted to say you finished the book, you could say:

• terminei de ler o livro = I finished reading the book
• acabei de ler o livro = I just finished reading the book / I finished reading the book

Parar de + infinitive is the most common and natural way to say to stop doing something.

So:

• parei de ler = I stopped reading

You could say other things, but they sound different:

• interrompi a leitura = I interrupted my reading
  This is more formal.
• parei na página 50 = I stopped at page 50
  This focuses on the exact page number.
• dei uma pausa na leitura = I took a break from reading
  This emphasizes the pause.

In your sentence, parei de ler is simple and very natural.

Yes, but it changes the tone slightly.

For example:

• Eu uso um marcador de página... = I use a bookmark...
• Eu uso meu marcador de página... = I use my bookmark...

Adding meu makes it more specific: a particular bookmark that belongs to me.

With a página, Portuguese usually does not need a possessive here, because the meaning is already clear. Saying minha página would sound odd in this context, since the page is part of the book, not something you own personally in that sense.

Yes, it is natural and correct. But a Brazilian might also say it in simpler ways, such as:

• Eu uso um marcador de página para não perder onde parei de ler.
• Eu uso um marcador para não perder a página onde parei.
• Eu uso um marcador para não perder meu lugar no livro.

Your original sentence is grammatical and clear. Some of these alternatives may sound a little more conversational.

Exactly. Para introduces the purpose of using the bookmark.

Structure:

• Eu uso um marcador de página = main action
• para não perder a página em que parei de ler = purpose

So the logic is: I use a bookmark for the purpose of not losing the page where I stopped reading.

This purpose clause with para + infinitive is extremely common in Portuguese.