Learn Spanish (Latin America): El libro es gratis en la tienda.

While this lesson focuses on adjectives that change their -o or -a endings to match the noun's gender, gratis (free) is an exception. It is an "invariable" adjective, meaning its spelling never changes. You will always use gratis regardless of whether the noun is masculine, feminine, singular, or plural.

Why do we use es instead of está for "is" in this sentence?

We use es (from the verb ser) because a cost—even when that cost is zero—is considered a core characteristic or price of the item. Estar is generally reserved for physical locations (like where the book is sitting) or temporary conditions.

Why does en la tienda translate to "at the store" instead of "in the store"?

The Spanish preposition en is very versatile and means both "in" and "at". You can translate it as "in the store" or "at the store" depending on what sounds most natural in the English sentence—both are perfectly correct!

If I wanted to say "the free book" instead of "the book is free", would I say el gratis libro?

No, you would say el libro gratis. As you are learning in this lesson, descriptive adjectives in Spanish almost always come after the noun they describe, which is the opposite of how we do it in English.