"You are frank."
Translation:Vous êtes francs.
Why does the adjective use the plural form (franches) for the vous conjugation?
Vous is the formal word for you or a word to address a group directly like "You people are frank" in English. Which is why it's conjugations are plural.
I have this mental picture of France being founded by tribes of blunt-speaking men...
What is the plural masculine form of " franc "? Is it " francs "? For example 'Vous êtes francs '.
Yes, exactly. When in doubt, you can usually find the variations in a dictionary. In this case:
masculine singular: franc
masculine plural: francs
feminine singular: franche
feminine plural: franches
I'm glad to know the answer to this question. Because it just does. My notes include a list of all the adjectives I have been exposed to (whew!) along with whether they are invariable or if they have a feminine form. It is just one of the things we have to learn.
Vous is more formal, and I'm not sure but I think that it can be plural too, whereas tu is neither. This is, however, only what I have heard, and you should ask a native speaker to be sure.