Learn Hardcore Spanish (Spain): Tengo cinco libros en mi cuarto.

In Spanish, with exact numbers you normally don’t use an article: Tengo cinco libros… means you have exactly five. If you say unos cinco libros, it means “about five.” Use los cinco libros only when you mean a specific known set: “the five books.”

Can I say Yo tengo… or should I drop yo?

Both are correct. Spanish usually drops subject pronouns, so Tengo cinco libros… is the default. Add Yo tengo… for emphasis or contrast, e.g., Yo tengo cinco, pero ella no tiene ninguno.

What’s the difference between tengo and hay in this context?

Tengo expresses possession: the books are yours. Hay expresses existence in a place, regardless of ownership: Hay cinco libros en mi cuarto = “There are five books in my room.” If they aren’t yours, prefer hay.

Why use en and not a or dentro de?

En marks static location (“in/at/on”) and is the normal choice: en mi cuarto. A indicates motion/goal: Voy a mi cuarto (“I’m going to my room”). Dentro de adds the nuance “inside of”: dentro de mi cuarto if you need to stress “inside.”