Si quieres, guárdate ese marcapáginas; yo ya tengo otro.

Because guárdate means something like keep it for yourself or take it for yourself, while guarda by itself usually just means keep/save/store.

Here, te adds the idea that the action is for you:

• Guarda ese marcapáginas = Keep that bookmark / Put that bookmark away.
• Guárdate ese marcapáginas = Keep that bookmark for yourself.

This use of te is very common in Spanish when someone is offering or suggesting that another person take something.

Because with affirmative commands in Spanish, object pronouns are normally attached to the end of the verb.

So:

• guarda + te → guárdate
• come + lo → cómelo
• lleva + la → llévala

If it were a negative command, the pronoun would go before the verb:

• No te guardes ese marcapáginas

So the position of te here is completely standard for an affirmative command.

When a pronoun is attached to an affirmative command, Spanish often adds a written accent to keep the original stress pattern.

Without the pronoun, the command is:

• guarda

When te is added, the word becomes longer:

• guardate

To show that the stress still falls on guár-, Spanish writes:

• guárdate

So the accent is not random; it helps preserve the correct pronunciation.

Si quieres literally means if you want.

In this kind of sentence, it works like a soft, polite way of making an offer or suggestion:

• Si quieres, guárdate ese marcapáginas
  = If you want, keep that bookmark.

It makes the sentence sound less direct and more friendly, similar to English if you like, if you want, or if you'd like.

Spanish demonstratives depend on how the speaker sees distance:

• este = this, near me
• ese = that, near you or not especially near me
• aquel = that over there, farther away

So ese marcapáginas suggests that bookmark rather than this bookmark.

In real conversation, the choice depends on context. If the bookmark is in the other person’s hand, or just not especially close to the speaker, ese makes sense.

Yes, it looks plural because it ends in -s, but marcapáginas is a singular noun here.

It is a compound noun, and its form is usually the same in singular and plural:

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

It is also masculine, which is why the sentence uses:

• ese marcapáginas
• otro

So even though it looks unusual, this is normal Spanish.

Yes, ya tengo otro would also be correct.

Spanish often drops subject pronouns because the verb already shows who the subject is. But yo can be included for emphasis or contrast.

Here:

• yo ya tengo otro

gives a slight contrast, something like:

• I already have another one
• As for me, I already have another one

So yo is not required, but it helps underline the speaker’s situation.

Ya often means already, and that is the basic idea here.

So:

• yo tengo otro = I have another one
• yo ya tengo otro = I already have another one

In context, ya helps explain why the speaker is offering the bookmark to someone else: there is no need for it, because they already have one.

Depending on tone, ya can also suggest as it is or anyway, but already is the clearest translation here.

Because Spanish often leaves out a noun when it is obvious from context.

• yo ya tengo otro means
• yo ya tengo otro marcapáginas

This is exactly like English I already have another one, where one replaces the noun.

Since marcapáginas is masculine singular, the replacement word is otro.

Yes. Quieres and guárdate are informal tú forms.

Informal:

• Si quieres, guárdate ese marcapáginas; yo ya tengo otro.

Formal:

• Si quiere, guárdese ese marcapáginas; yo ya tengo otro.

The meaning stays the same, but usted sounds more formal or more distant.

The semicolon links two closely related parts:

• Si quieres, guárdate ese marcapáginas
• yo ya tengo otro

The second part explains the first: the speaker is offering the bookmark because they already have another one.

A full stop would also be possible:

• Si quieres, guárdate ese marcapáginas. Yo ya tengo otro.

A semicolon simply shows a strong connection between the two clauses while keeping them in one sentence.

Yes, it sounds natural in Spain.

A few notes:

• marcapáginas is a normal word in Spain for bookmark.
• guárdate sounds natural when offering something for someone to keep.
• The overall tone is friendly and informal.

In Spain, you might also hear slight variations depending on style, such as:

• Si quieres, quédate ese marcapáginas; yo ya tengo otro.
• Si quieres, puedes quedarte ese marcapáginas; yo ya tengo otro.

But the original sentence is perfectly good Spanish.