Guardo el marcapáginas dentro de mi libro para no perderlo.

Here guardar means something like to keep, to put away, or to store.

It is a good choice because it suggests putting something somewhere safely. In this sentence, the idea is not just placement, but placement so it won’t get lost.

Compare:

• guardar = to keep/put away/store
• poner = to put/place
• dejar = to leave

So:

• Guardo el marcapáginas... = I put/keep the bookmark there safely
• Pongo el marcapáginas... = I place the bookmark there
• Dejo el marcapáginas... = I leave the bookmark there

All can be possible in some contexts, but guardar best matches the idea of keeping it safe.

Yes, marcapáginas is singular here, even though it ends in -s.

In Spanish, some nouns ending in -s are still singular. The article tells you whether it is singular or plural:

• el marcapáginas = the bookmark
• los marcapáginas = the bookmarks

So the word form often stays the same, and the article shows the number.

It is also masculine, so Spanish uses el and later lo.

The accent mark shows where the stress goes: marcapáginas.

Without the accent, Spanish stress rules would make you stress a different syllable. The written accent keeps the stress on pá.

So you pronounce it roughly like:

• mar-ca-PÁ-gi-nas

This is very common in compound words where the normal stress needs to be preserved.

Spanish often uses the simple present in places where English might use either:

• I keep
• I put
• I am putting

So guardo can sound natural both as:

• a habitual action: I keep the bookmark inside my book
• something happening now: I’m putting the bookmark inside my book

If you wanted to strongly emphasize right now, at this moment, you could say:

• Estoy guardando el marcapáginas...

But the simple present is very normal here.

Yes, en mi libro is also possible.

The difference is mainly one of precision:

• dentro de mi libro = inside my book
• en mi libro = in my book, which can also mean inside it, but is a bit less explicit

Using dentro de makes the physical location clearer. It emphasizes that the bookmark is inside the book, not just associated with it.

Another natural option would be:

• entre las páginas de mi libro = between the pages of my book

In Spanish, the short possessive adjectives normally replace the article.

So you say:

• mi libro = my book
• tu libro = your book
• su libro = his/her/your book

Not:

• el mi libro ❌

Spanish does have longer possessive forms like:

• el libro mío

But that structure is used differently, usually for emphasis, contrast, or style. In a normal sentence like this, mi libro is the standard form.

This is a very common Spanish structure:

• para
  • infinitive = in order to / so as to

So:

• para perderlo = in order to lose it
• para no perderlo = in order not to lose it / so I don’t lose it

It expresses purpose.

So the logic of the sentence is:

• I keep the bookmark inside my book
  for the purpose of not losing it

Lo refers to el marcapáginas.

That is why it is:

• lo = masculine singular direct object pronoun

Because marcapáginas is masculine and singular, the pronoun is lo.

So:

• perder el marcapáginas = to lose the bookmark
• perderlo = to lose it

Because perder is an infinitive.

In Spanish, object pronouns are often attached to the end of an infinitive:

• perderlo = to lose it
• hacerlo = to do it
• verla = to see her/it

With an infinitive, this is the normal position.

You could also say the noun instead:

• para no perder el marcapáginas

But using perderlo sounds more natural because it avoids repetition.

Because marcapáginas is the direct object of perder.

You lose something, so Spanish uses a direct object pronoun:

• lo / la / los / las

Not:

• le / les

So:

• perderlo = lose it ✔
• perderle = not correct here ✘

Even in varieties of Spanish where le is sometimes used for male people, it is not used for a thing like marcapáginas.

A little, yes, but this version is very natural:

• Guardo el marcapáginas dentro de mi libro para no perderlo.

You could also say:

• Guardo dentro de mi libro el marcapáginas para no perderlo.
• El marcapáginas lo guardo dentro de mi libro para no perderlo.

But these have different emphasis and may sound less neutral.

The original order is good because it is clear:

1. action — Guardo
2. object — el marcapáginas
3. place — dentro de mi libro
4. purpose — para no perderlo

Yes. A few natural alternatives are:

• Guardo el marcapáginas en mi libro para no perderlo.
• Dejo el marcapáginas dentro de mi libro para no perderlo.
• Pongo el marcapáginas dentro de mi libro para no perderlo.
• Lo guardo dentro de mi libro para no perderlo.

These are all understandable, but the original sentence is nice because guardar fits the idea of keeping it safe especially well.