one of the interrogative phrases is in the informal form (tu) while the other is in the formal (vous). comment vas-tu? (informal) comment allez-vous? (formal) the meaning for both phrases is identical (id est no semantic shifts)

It is the same difference that the Portuguese make between "TU" and "VOCÊ". The former is informal, used among friends and family. The latter is formal. I understand that "tu" is almost never used to Brazil. Is this true?

the difference is very simple, pay attention please :) when you say, comment vas tu? ; usually is a question for a friend ( singular)=(one friend) or a member of familly . but when you say, comment allez-vous? this is very formal question, and it is usually for someone we have just met, someone we don't know, or someone that we respect so much, (a boss for example), and it is strongly used in formal letters, because people you are sending it to someone you don't know, and respecting them is a must . (but ' comment allez vous? ) is plural, so you have to use it when you are asking more than one person , it doesn't matter if they are familly , friends, it doesn't matter if you know them or not, you have to use it simply because they are plural ( more than 2 persons) I hope you can find this very useful :) yours Med

the difference is very simple, pay attention please :) when you say, comment vas tu? ; usually is a question for a friend ( singular)=(one friend) or a member of familly . but when you say, comment allez-vous? this is very formal question, and it is usually for someone we have just met, someone we don't know, or someone that we respect so much, (a boss for example), and it is strongly used in formal letters, because people you are sending it to someone you don't know, and respecting them is a must . I hope you can find this very useful :) yours Med