Learn Portuguese (Portugal): O livro está dentro da mala.

The phrase for "inside" is dentro de. Because mala (bag) is a feminine noun, we need the word for "the" in front of it: a. In Portuguese, de + a combine to make the contraction da. So, dentro da mala literally means "inside of the bag".

Could I use na instead of dentro da here?

Yes! You could say O livro está na mala (em + a = na), which simply means "The book is in the bag". Using dentro da emphasizes that it is specifically "inside of" the bag, much like the difference between "in" and "inside" in English.

Why do we use está for "is" instead of é?

Portuguese has two verbs for "to be": ser and estar. We always use estar (here as está) to talk about physical location, regardless of whether that location is temporary or permanent. Since we are stating where the book is located, está is the correct choice.

Does mala mean "bag" or "suitcase"?

Mala is a very versatile word that can mean both! Depending on the context, it might be a handbag, a piece of luggage for travel, a briefcase, or even the trunk of a car. Here, translating it as "bag" makes perfect sense.