1.) "Eres" is a tense from the verb "ser," which is a permanent verb that means "to be." Normally, a person will always be short. Also, "eres" is the only verb conjugation that correctly completes the sentence.
Eres: you are
somos: we are (both)
son: they are
soy: I am
2.) "Son" means "they are," permanently. They are ALWAYS active... remember that this would not make sense if they are SOMETIMES active, which would be "estan" instead of "son."
