Para colocar o verbo to be em forma negativa, você coloca o sujeito + verbo to be (conjugado) + not + o resto da frase. por exemplo: I am a girl -> I am not a girl You are a student -> You are not a student. He is a soccer player -> He is not a soccer player We are students -> We are not students. e assim por diante.
Há a forma negativa I am not - I'm not You are not - You aren't He is not - He isn't