Me regaló un marcapáginas de papel con flores azules.

Me is the indirect object pronoun meaning to me.

So Me regaló... literally means something like He/She gave ... to me as a gift.

With regalar, Spanish often uses an indirect object pronoun:

• Me regaló = he/she gave me as a present
• Te regaló = he/she gave you as a present
• Nos regaló = he/she gave us as a present

Spanish often leaves out subject pronouns when they are clear from the verb form or from context.

Regaló is a third-person singular form, so the subject could be:

• él = he
• ella = she
• usted = you (formal)

So the sentence could mean:

• He gave me a paper bookmark with blue flowers
• She gave me a paper bookmark with blue flowers
• You (formal) gave me a paper bookmark with blue flowers

Context tells you which one it is.

Regaló is the preterite of regalar.

The preterite is used for a completed action in the past:

• Me regaló un marcapáginas = he/she gave me a bookmark

Compare:

• me regala = he/she gives me / is giving me
• me regalaba = he/she used to give me / was giving me
• me regaló = he/she gave me

So this sentence refers to one completed past event.

The accent mark shows the correct stress: re-ga-LÓ.

It also helps distinguish it from regalo, which can mean:

• I give as a gift
• gift / present (as a noun)

So:

• regalo = I give / a gift
• regaló = he/she/you-formal gave

That written accent is very important here.

Not exactly.

• dar = to give
• regalar = to give as a gift / to give someone a present

So Me regaló un marcapáginas is more specific than Me dio un marcapáginas. It tells you that the bookmark was a gift, not just something handed over.

Marcapáginas means bookmark and is a compound noun, formed from:

• marcar = to mark
• páginas = pages

In standard Spanish, it is usually treated as a masculine noun:

• un marcapáginas
• el marcapáginas

Its plural is usually the same form:

• dos marcapáginas
• los marcapáginas

So it is common to say:

• un marcapáginas
• tres marcapáginas

Because it is a fixed compound noun in Spanish, not just a random combination of words.

Spanish often creates nouns like this:

• abrelatas = can opener
• sacacorchos = corkscrew
• paraguas = umbrella
• marcapáginas = bookmark

So in modern standard Spanish, marcapáginas is normally written as one word.

De papel means made of paper or paper as a material.

When Spanish expresses material, it often uses de + material noun without an article:

• una mesa de madera = a wooden table
• una botella de vidrio = a glass bottle
• un marcapáginas de papel = a paper bookmark

Del papel would mean of the paper and would usually refer to some specific paper, which is not the idea here.

It describes the marcapáginas.

So the bookmark is:

• de papel = made of paper
• con flores azules = with blue flowers

In natural English, this often means the bookmark has blue flowers on it or is decorated with blue flowers.

Because Spanish adjectives usually agree in number with the noun they describe.

Here:

• flores is plural
• so azul becomes azules

Compare:

• una flor azul = a blue flower
• dos flores azules = two blue flowers

So azules is plural because flores is plural.

Some Spanish adjectives change for both gender and number, but azul only changes for number.

So:

• masculine singular: azul
• feminine singular: azul
• masculine plural: azules
• feminine plural: azules

That is why you get:

• flor azul
• flores azules

The adjective matches the noun in number, but its masculine and feminine singular forms are the same.

Yes. That is correct.

A mí adds emphasis:

• Me regaló... = He/She gave me...
• A mí me regaló... = He/She gave me..., or It was me that he/she gave... to

The longer version is not necessary in a neutral sentence, but it is very common when you want contrast or emphasis:

• A mí me regaló un marcapáginas, y a mi hermana le regaló un libro.

Yes, it sounds natural in Spain.

A speaker from Spain would readily understand and use:

• Me regaló un marcapáginas de papel con flores azules.

Also, marcapáginas is a very normal word in Spain for bookmark.