Incorrect spelling


Incorrect spelling, explanation: to create this adjective we have to add a suffix -ful to a verb. The suffix is spelled with just one l at the end, which may not be obvious. This is why usefull is an incorrect spelling and the correct form is useful – with one consonant l.

Correct spelling


Correct spelling, explanation: adjective-creating suffix -ful gives the meaning of full of, characterized by, and is always spelled with one l. This may not be obvious since it’s a different spelling from the word full. Thus useful will also be written with just one consonant l. Usefull is the wrong form.

Definition of useful:
adjective, being convenient, advantageous
My new kitchen supplies are so useful, I love them.
Why do you always buy these unnecessary gadgets? They aren’t useful at all.