The negation particle personne is very similar to rien and both are different from pas or plus. The difference lies in the fact that personne ("nobody") and rien ("nothing") can be used as subjects, as well as objects in a negated sentence, whereas pas and plus can't. That's why the placement of personne (and rien) in a sentence is determined by their grammatical function. They don't just bracket the negated verb as pas and plus would.
Personne n'a annulé. -- Nobody cancelled. Personne is the subject.
Il n'y a personne qui a annulé. -- There is nobody who cancelled. Here, personne is the COD.
Je n'ai vu personne. -- I saw nobody. (Personne is the COD)
Je ne le dirai à personne. -- I'll tell it to nobody. (Personne is the indirect object.)
Rien ne s'est passé. -- Nothing happened. (Rien is the subject.)
Je ne sais rien. -- I know nothing. (Rien is the COD.)
