Learn Hardcore Spanish (Spain): Guardo mis libros en el cajón del escritorio.

guardar is a regular –ar verb meaning “to put away,” “to keep,” or “to store” with the nuance of safekeeping or placing something in its designated spot. By contrast, poner simply means “to put” or “to place” (without implying you’re tidying or storing), and almacenar often suggests storing large quantities or bulk items (e.g. in a warehouse or storage room).

Why do we use mis instead of mi before libros?

Spanish possessive adjectives agree in number with the noun they modify. You use mi for one item (mi libro) and mis for more than one (mis libros).

What does the preposition en indicate here? Could I use another preposition like a or sobre?

Here, en indicates location: “in” or “inside.”

a would imply movement toward (“I go to the drawer”) rather than location.
sobre means “on top of,” so sobre el cajón would mean the books are on the drawer, not inside it.

What exactly does el cajón del escritorio refer to?

It literally means “the drawer of the desk,” i.e. the desk drawer. You need both words to specify which drawer you’re talking about—just el cajón could be any drawer, and el escritorio by itself is the whole desk.

Why is del used instead of writing de el?

In Spanish, de + el contracts to del whenever de precedes the masculine singular definite article el. Saying de el is grammatically incorrect—you must say del.

Could I say en mi escritorio instead of en el cajón del escritorio? How would the meaning change?

Yes, but en mi escritorio is more general: it could mean “on my desk” or “somewhere within the desk area.” En el cajón del escritorio specifies that the books are inside the desk’s drawer.

Can I omit mis and just say Guardo libros en el cajón del escritorio? What about replacing mis libros with los?

You can omit mis if context already makes clear whose books they are, but it loses specificity. After mentioning los libros, you can use the direct object pronoun los: Los guardo en el cajón del escritorio means “I keep them in the desk drawer,” with los referring back to los libros.

Why is the sentence in the simple present guardo instead of the present progressive estoy guardando?

The simple present (guardo) describes a habitual action or general fact: “I keep/store my books….” The present progressive (estoy guardando) would emphasize that the action is happening right now as you speak (e.g. “I am (in the process of) putting my books into the drawer”).