Reiterando, basta o verbo auxiliar em casos como:
Can you help me?
Yes, I can.
Are you going now?
Yes, I am.
Should we stop now?
Yes, we should.
Frequentemente, a pergunta aparece como uma simples afirmativa. Mas mesmo assim a rsposta utiliza o auxiliar ou o verbo que corresponde a forma interrogativa, tivesse essa sido usada:
You went to the school? The teacher was there? (Did you go...? Was she...?)
Yes, I did. And, yes, she was.
He called before I came home? He was happy, I guess. (Did he call...? Was he...?)
Yes, he did. And, yes, he was.