In Spanish, like in English, we can make questions in a negative form. In this case we express our surprise or suggest that the action will not be completed. Compare:
¿Vas al cine? – I think you will go
¿No vas al cine? – I think you will not go (and possibly I am surprised)
Like in English, sí always means yes:
– ¿Vas al cine?
– Sí = Yes, I will go– ¿No vas al cine?
– Sí = Yes, I will go
and no means not:
– ¿No vas al cine?
– No = No, I won´t
If we use a verb in the answer, we must repeat no:
– ¿No vas al cine?
– No, no voy
Unlike Portuguese, Spanish never answer with only the verb form:
– ¿No vas al cine?
– *Voy
And, please, never answer repeating the questions!
– ¿No vas al cine?
INCORRECTO:– *No voy al cine
CORRECTO: No / No, no voy