Learn Spanish (Latin America): Usamos el grano tostado.

Tostado comes from the verb tostar, which means to roast or to toast. To form the regular past participle of an -ar verb, you drop the -ar ending and add -ado. In this sentence, it is used to mean roasted.

Is tostado acting as a verb or an adjective here?

It is acting as an adjective. Past participles are very commonly used as adjectives to describe the state of a noun. Because it is describing el grano, it is placed right after the noun, just like a normal descriptive adjective in Spanish.

If we were talking about multiple beans, would tostado change?

Yes, it would. Because it is acting as an adjective here, it must agree with the noun in both gender and number. If we said the roasted beans (los granos), the participle would add an -s to match: los granos tostados.

How do we know usamos means we use and not we used?

Out of context, you do not! For regular -ar verbs like usar, the present tense we form and the past tense (preterite) we form are exactly the same: usamos. You usually rely on the rest of the conversation or time markers to know which tense is meant.