But Nefal, while it is often passive, can also be reflexive. For example, you can say ani nishmar, and it means either “I am guarded,” as in someone is guarding me, or “I am being careful,” as in I am guarding myself. For this, Hitpael would serve as well for this reflexive usage, but lacks the ambiguity. Ani mishtamer is “I am guarding myself.” Period. No other meaning is possible. Paul, on the other hand, is unambiguous. Ani shamur means “I am guarded” as in somebody else is guarding me and no room for reflexive doubts. So you can be either very precise using other forms and voices, or deliberately vague using Nefal.
You are absolutely right about nefal and the other verb tenses. Like I said, if you were sitting next to me in class, I would be copying your homework.