Vous devez etre un homme
How does this sentence make sense? Doesn't "vous" imply that you're speaking to more than one person?
In French, there are two forms of 'you' when talking to a single person: 1. 'Vous' which is the polite form, used in situations where showing respect is appropriate 2. 'tu' which is the more casual form, used with friends and familiar people
The confusing part is that 'vous' also means 'you' when talking to multiple people. Sometimes it can be tricky to decide which is meant - you have to judge by context and the form of the verb.
Yes if it is more then one person or if you meet someone for the first time you'd say vous instead of Tu. Also if you are talking with a teacher or an adult you'd use vous unless if the adult is related to you then you would use Tu.