I would have answered that this is the wrong question. "Can" and "may" are English words, so the distinction between them exists only in English.
If the original question (now two years old) was about the expressing en Esperanto the difference between:
- How may I help you?
- How can i help you?
My first question would be - what is the difference in meaning that you perceive in English? We need to know what the difference is before talking about how to express that difference in Esperanto.
Not only is inversion (or any changing of word-order) not needed in Esperanto to form a question, changing the word order in Esperanto usually makes no difference to the meaning of a sentence. For instance, "Mi trinkas bonan kafon", "Bonan kafon mi trinkas", and "Trinkas mi bonan kafon" all mean the same thing.
"helpo" is the noun "help", as in "Thank you for your help." ("Dankon pro via helpo.") When you put an n on the end of "helpo", ("helpon") you make the noun accusative, that is it is the thing that is having something done to it, as in "He requested help" (Li petis helpon). "He" is doing something, requesting. "Help" is having something done to it - it is being requested.
"helpi" is "to help", the infinitive form of the verb. "Li volas helpi vin" for example, means "He wants to help you".
Does that answer your question? I hope so.
"Helpi" is the infinitive form of the verb, the same as the English "to help". "Helpas" is the present tense of the same verb, so "Mi helpas" means "I help", "Li helpas" means "he helps", and so on.