Adding -ful or -fully

The suffix -ful can form nouns or adjectives, like plateful or cheerful. People sometimes make the mistake of spelling this type of word with a double l at the end. Note that it is always spelled with just one -l:

dreadful, faithful, skillful, powerful
cupful, mouthful, spoonfuls
The related ending –fully forms adverbs. Remember that this suffix is always spelled with two l’s:
dreadfully, faithfully, skillfully, powerfully

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Grammar and usage