Harvard or no Harvard, I have to disagree with the grammar of this teacher. Don’t get me wrong, she is an excellent Spanish teacher and a really nice person, but I think it’s time to get this straight.
So in Spanish, we are learning how to distinguish the subjunctive form and the regular form. Now listen up. If the command is negative, you use the subjunctive form. Take the verb doubt (dudar), for example. Since dudar means to doubt, that is negative. If I doubt that he was a good student, that is a negative statement. “NO WAY!!” claims the completely wrong teacher (who probably couldn’t hear my question in class). Now If I don’t doubt (no dudar), that is positive. If I don’t doubt that you are nice, that means that you are nice. The same thing with the verb creer (to believe). If I believe that you will do something, that is positive (taking the regular present verb form) If I do not believe (no creer) that is negative and takes the subjunctive form.
That is my point. Please correct me if I am wrong.