Learn Spanish (Latin America): Hay muchos libros en la caja.

In Spanish, the preposition en is very versatile and commonly means 'in', 'on', or 'at'. For locations, like books being inside a box, en is usually all you need. If you really wanted to emphasize the 'inside' part, you could use dentro de, but en is the most natural choice here.

Why does hay mean 'there are'? Doesn't it mean 'there is'?

Hay is a special, very common word that means both 'there is' and 'there are'. Unlike English, it doesn't change when you are talking about multiple things. So, you use the exact same word for hay un libro (there is a book) and hay muchos libros (there are many books)!

Why does muchos end in -os?

In Spanish, describing words have to match the gender and number of the thing they describe. Because libros (books) is masculine and plural, the word for 'many' has to match it, making it muchos. If you were talking about a feminine word, like the boxes themselves, you would say muchas cajas.

How do I know to use la with caja?