Hagas lo que hagas, no olvides el marcapáginas en la biblioteca.

Literally, it is something like Do-subjunctive what do-subjunctive, which sounds odd in English.

As a whole, hagas lo que hagas is a fixed Spanish pattern meaning whatever you do or no matter what you do. It is not meant to be translated word by word.

You will see the same pattern with other verbs:

• Digas lo que digas = whatever you say
• Vayas donde vayas = wherever you go
• Pase lo que pase = whatever happens

So the important thing is to learn it as an idiomatic structure.

Because Spanish often uses this repetition in expressions meaning no matter what... or whatever....

The pattern is:

subjunctive verb + lo que + same subjunctive verb

So:

• hagas lo que hagas
• digas lo que digas
• escribas lo que escribas

The repetition creates the idea of any possible action, not one specific action.

Lo que is a very common structure meaning what, that which, or in patterns like this, whatever.

Here, lo is not a normal masculine article. It is a neuter article, used for abstract or unspecified things. So lo que refers to whatever thing / whatever action.

That is why hagas que hagas would be wrong here. You need lo que in this expression.

Because the sentence is talking about an indefinite, non-specific possibility: whatever you do, not one known action.

Spanish normally uses the subjunctive in this kind of open-ended relative structure. The idea is not the thing that you do, but any possible thing you may do.

Compare:

• Haces lo que haces = you do what you do
  This sounds like a statement about a known fact.
• Hagas lo que hagas = whatever you do
  This expresses indefiniteness and uses the subjunctive.

So the subjunctive is important for the meaning.

Because no olvides is a negative tú command: don’t forget.

In Spanish, negative commands for tú use the present subjunctive:

• Olvida = forget! (positive tú command)
• No olvides = don’t forget! (negative tú command)

The other forms would mean something else:

• no olvidas = you don’t forget (a statement, not a command)
• no olvida = he/she/it doesn’t forget or you don’t forget in usted meaning, depending on context

So no olvides is exactly the right form for telling one person informally not to forget something.

It is addressing one person informally, so tú.

You can tell from:

• hagas
• no olvides

Both are tú subjunctive forms.

Other versions would be:

• Haga lo que haga, no olvide... = formal singular (usted)
• Hagáis lo que hagáis, no olvidéis... = informal plural in Spain (vosotros)
• Hagan lo que hagan, no olviden... = plural formal / Latin American plural (ustedes)

Since you asked about Spanish from Spain, it is especially useful to notice the vosotros version too.

Marcapáginas means bookmark.

It is a compound noun, formed from the idea of marking pages. Spanish has many nouns like this, often built from a verb + noun:

• abrelatas = can opener
• sacacorchos = corkscrew
• lavavajillas = dishwasher
• marcapáginas = bookmark

So even if it looks long, it is a very normal type of Spanish word.

Because the gender of the whole compound noun is what matters, not the gender of one part inside it.

Many Spanish compound nouns of the type verb + noun are masculine:

• el sacacorchos
• el lavavajillas
• el marcapáginas

So you say el marcapáginas, even though la página by itself is feminine.

Also, marcapáginas has the same form in singular and plural:

• el marcapáginas
• los marcapáginas

The article tells you whether it is singular or plural.

Yes. That would also be very natural.

Spanish often allows both:

• olvidar algo = to forget something
• olvidarse de algo = to forget about something / to forget something

So both of these work:

• No olvides el marcapáginas
• No te olvides del marcapáginas

The version in your sentence is slightly more direct and compact. The reflexive version is also very common in everyday speech.

En can mean both in and at, depending on context.

So en la biblioteca could be understood as:

• in the library
• at the library

With the verb olvidar, there is also a possible nuance of leaving something behind there by mistake. So the sentence could suggest:

• Don’t forget the bookmark when you’re at the library.
• Don’t leave the bookmark behind in the library.

Usually the broader context tells you which meaning is intended.

Because Hagas lo que hagas is an introductory clause before the main command:

• Hagas lo que hagas,
• no olvides el marcapáginas en la biblioteca.

In English, you would also normally separate Whatever you do, ... with a comma.

So the comma helps show the structure clearly and makes the sentence easier to read.