Guárdate ese marcapáginas y no dobles la contraportada, que luego se nota.

Because guardarse algo is a very common Spanish pattern meaning to keep something for oneself, to put something away, or to hold on to it.

So:

• guarda ese marcapáginas = put away / keep that bookmark
• guárdate ese marcapáginas = keep that bookmark for yourself / put it away

The -te is the reflexive/indirect pronoun for tú. In everyday Spanish, this often adds a personal nuance, not always a strongly literal reflexive meaning.

This kind of construction is very common in speech:

• Cómete esto = eat this up
• Llévate eso = take that away / take that with you
• Guárdate eso = keep that / put that away

So here it sounds natural and conversational.

When an affirmative command takes attached pronouns, Spanish often adds a written accent to keep the original stress.

Without the pronoun:

• guarda

With -te attached:

• guárdate

The accent shows that the stress still falls on guár-.

This is a general rule with affirmative commands + pronouns:

• dime
• siéntate
• llévatelo
• guárdate

Ese means that.

Spanish has a three-way contrast:

• este = this, near the speaker
• ese = that, near the listener or not especially close to the speaker
• aquel = that over there, farther away

So ese marcapáginas means that bookmark.

In real speech, the difference between este and ese is sometimes less strict than textbook explanations suggest, but here ese sounds like the speaker is referring to a bookmark the other person has or is using.

Marcapáginas means bookmark. It is one of those Spanish compound nouns that look plural because they end in -s, but they can still be singular.

So:

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

The article tells you whether it is singular or plural.

It comes from:

• marcar = to mark
• páginas = pages

Literally, something like page-marker.

Because negative commands in Spanish use the present subjunctive, not the normal present tense.

For tú:

• affirmative command: dobla
• negative command: no dobles

So:

• Dobla la contraportada = Fold the back cover
• No dobles la contraportada = Don’t fold the back cover

This is a key pattern in Spanish:

• habla / no hables
• come / no comas
• vive / no vivas

Here doblar means to bend or to fold.

In the sentence, it refers to physically creasing the book’s back cover.

Doblar can have several meanings depending on context:

• fold/bend something
• turn a corner
• double in some contexts
• dub a film or actor’s voice

But in this sentence, the physical meaning to bend/fold is the one that matters.

Contraportada usually means the back cover of a book, magazine, or similar item.

So:

• portada = front cover
• contraportada = back cover

Depending on context, English might translate it as:

• back cover
• sometimes rear cover

Here it clearly refers to the book’s outer back cover.

Spanish often uses the definite article (el, la, los, las) where English might prefer a possessive like his, her, its, your.

So instead of saying:

• don’t bend its back cover

Spanish naturally says:

• no dobles la contraportada

The ownership is understood from context: we know it is the back cover of the book being handled.

This is very common in Spanish:

• Me duele la cabeza = My head hurts
• Abre la puerta = Open the door
• No cierres los ojos = Don’t close your eyes

Here que introduces the reason, something like because or it’s just that.

So:

• no dobles la contraportada, que luego se nota means
• don’t bend the back cover, because it shows afterwards
• don’t bend the back cover, because you can tell later
• don’t bend the back cover, because the mark shows

This que is very common in spoken Spanish. It often sounds more conversational than a more explicit porque.

Notarse means to be noticeable, to show, or to be obvious.

So se nota here means:

• it shows
• it’s noticeable
• you can tell

In context, the idea is that if you bend the back cover, the crease will remain visible later.

A few examples:

• Se nota que está cansado = You can tell he’s tired
• La mancha se nota mucho = The stain is very noticeable
• Luego se nota = You can tell afterwards / It shows later

Here luego means later or afterwards.

So que luego se nota means the damage becomes visible later on.

In Spain, luego very commonly means later or afterward. In some other varieties of Spanish, it can also be used in ways that overlap with then.

Here the sense is:

• you bend it now
• later the crease is still visible

It is informal, because it uses the tú forms:

• guárdate
• no dobles

A formal version addressed to usted would be:

• Guárdese ese marcapáginas y no doble la contraportada, que luego se nota.

The original sounds like something a friend, parent, teacher, librarian, or shop worker might say in a casual or everyday setting.

Yes, it sounds natural and very much like spoken Peninsular Spanish.

A few things that make it sound natural:

• guárdate gives it a conversational tone
• ese marcapáginas is straightforward and idiomatic
• que luego se nota is a very everyday spoken way to explain the reason

It sounds like someone telling another person to treat a book properly:

• keep the bookmark
• don’t fold the cover
• because the damage will show later

So overall, it is a realistic, colloquial sentence.