A yes/no question is answered by repeating the verb, not with sim/não. A bare 'sim' sounds curt or foreign.
- Tens fome? — Tenho.
- Are you hungry? — I have.
- Já almoçaste? — Já.
- Have you had lunch? — Already.
- Queres vir? — Quero.
- Do you want to come? — I want to.
