A suffix is a letter or group of letters added to the end of a word to make a new word.
It is often hard to know whether or not to double the final letter of a word before adding a suffix, but there are rules to help.
The word chop has a short vowel sound ('o') just before the final consonant, so the 'p' must be doubled before adding a suffix.
chop chopped chopping
The word sleep has a long vowel sound ('ee') just before the final consonant, so the 'p' is not doubled.
sleep sleepy sleeping
The final consonant is not doubled if the letter just before it is also a consonant.
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