I think "eres" is second person singular, "es" is third person singular. Like this: "Él es un niño." = He is a boy; "Eres un niño." = You are a boy; It doesn't have anything to do with the gender.

It does mean 'You', but more formally. It's just how Romanic Languages work - you use third person singular to be more formal when speaking to a stranger or someone important. E.g. you use "Tu eres un hombre" (you are a man) to say to someone you know (a friend for example) and "Usted es un hombre" (You are a man) to say the same to a stranger.

Exactly, sommerlied.

But why "Usted es una niña." ? Isn't it second person singular?

I think that when you use Usted it's more formal. And Usted is third person singular, so 'es' is the correct form of the verb 'to be'.

Usted is third person singular? I thought it means "You"?

Understood. Thank you very much. It's very helpful. :)

will "tu es" be considered incorrect?