Correct spelling


Correct spelling, explanation: the reason for this spelling is the suffix -ful (meaning full of, characterized by) is written with just one l (which may be confusing due to its meaning). Thus help + ful is helpful, not helpfull.

Definition of helpful:
adjective, characterized by willingness to help
She was very helpful and explained to me the whole homework.
Assistants in this shop aren’t much helpful and sometimes they are rude. I don’t go there any more.

Incorrect spelling


Incorrect spelling, explanation: the spelling of this word may be confusing, because of the meaning that suffix -ful gives to an adjective (full of or characterized by), but this suffix is spelled only with one l. This is why helpful is the correct form. Helpfull is a common misspelling.