Learn Spanish (Spain): Compramos libros en la tienda.

The ending of the verb gives it away! The base verb is comprar (to buy). When you change the ending to -amos, it specifically means "we buy." Because compramos already has "we" built into it, you don't need to add the pronoun nosotros (we).

How do I make this "we" form for other regular verbs?

For regular verbs that end in -ar, you just chop off the -ar and add -amos. So, comprar becomes compramos in this sentence, just like hablar (to speak) becomes hablamos (we speak).

Why does it say libros instead of los libros?

Just like in English, you can leave out the word for "the" (los) when you are talking about an unspecified amount of things. They are buying "books" in general, not specific books that were mentioned before.

The translation says "in the shop." Does en always mean "in"?

Not always! The Spanish word en is very flexible. Depending on the sentence, it translates to "in," "on," or "at." Saying en la tienda can mean "in the shop" or "at the shop" interchangeably.