Correct spelling


Correct spelling, explaination: the adjective useful is spelled with one l at the end, because it is made from the verb use and suffix -ful, which is spelled with just one l. This suffix gives the meaning full of or with features of; it used to be spelled -full, just like the word of its origin, but with time it was shortened. This is why usefull is a misspelling and useful is the correct form.

Definition of useful:
adjective, something being used for particular purpose; convenient, handy
Have you tried this new vocabulary check app? It’s very useful.
This knife is much more useful to chop onions.

Incorrect spelling


Incorrect spelling, explaination: overtime old suffix -full (now -ful) lost second l and became -ful. This is why, while giving the meaning full of, it is written with just one l (unlike its origin). Thus here the correc spelling is useful, not usefull. This is an often mistake, because the word full is spelled with double l.