Step 1

To tell someone not to do something:

     tú piensas ---> no pienses
     tú comes ---> no comas

     ¡No fumes!
      Don't smoke!
     ¡No corras!
      Don't run!
     ¡No duermas!
      Don't sleep!
     ¡No comas!
      Don't eat!
     ¡No vuelvas!
      Don't come back!
     ¡No pienses!
      Don't think!

No cierres la portezuela.
      Don't close the door.
No le des la llave.
      Don't give him the key.
No te enojes conmigo.
      Don't get upset with me.
¡No me hables de él!
      Don't talk to me about him!

We can also use nunca:

     Nunca camines por ese callejón.
      Never walk in that alley.