When the main words ends in "e," we keep the vowel "e" and add the suffix "ly."
Explanation:
As per the spelling rule
1) We just add -ly to words ending in a consonant.
For example:Slow+ly=Slowly,Week+ly=Weekly
2) We add -ly to words ending in -ful,
For Example: Ful+ly -fully,beautiful + ly = beautifully
3) We add -ly to words ending in -l
For example: accidental + ly = accidentally ,cool + ly = coolly
4) We keep the 'e'
For Example: complete + ly = completely ,definite + ly = definitely
However, there is an exemption to this rule.
For example: true = truly
In this case, we drop the letter "e" and add "ly."
we drop the 'e' in duly due + ly = duly