"Please help me!"
Translation:Bonvolu helpi min!
You can't have two imperatives in the same clause. In the first example "bonvoli" (to be of good will) is imperative so "helpi" needs to be in the infinitive. You could turn "helpi" into an adjective: "bonvole - 'goodwillingly'" and then use "helpu" - "Bonvole helpu min". In the second example "Mi petas" means "I request" and usually goes at the end of the request: "Helpu min, mi petas" - "Help me, I request". "Helpi" is the verb you want the other person to do and so it takes the imperative. In the two example you give, "helpi" is serving two different functions and so takes two different forms.