The following spelling rule explains that “likely” is spelled "likely” rather than “likly.” (1
When the suffix ends with a consonant, keep the "e" on the main word.
When the suffix begins with a consonant, keep the "e" on the main word.
When the main word ends in “e,” keep the "e" and add "ly."
O When the main words ends in a consonant then a vowel, keep the “e” and add "ly."