"Os sanduíches são gratuitos."

Translation: The sandwiches are free.

[ketoacidosis]

What's the difference between gratuitos and gratis? How do we know when to use which?

[Paulenrique]

Gratuito is an adjective. A pipoca é gratuita. (The popcorn is free). Grátis is an adverb; is related to a verb. A pipoca vem grátis / vem de graça. (Literally: the popcorn comes for free). As gratuito is an adjective, it follows the gender and number of a word.

[ketoacidosis]

Obrigado!

What about livre? The hints tell me it means "free." I'm guessing this is similar to liberated/ released?

[Paulenrique]

"Livre" is related to freedom (liberdade) "nobody is free" = "ninguém é livre" (you translated this way because you are not meaning price, but "the quality of having freedom")

[ketoacidosis]

Obrigado novamente!

[pietvo]

As they say "free beer" vs. "free speech" (FSF)

[Chaimalka]

Why can't I say the sandwiches are gratis?

[Paulenrique]

"grátis" is an adverb, not an adjective.