RULE 1
Words ending in a consonant add LY:
-slow to slowly
-friend to friendly
-week to weekly
-quiet to quietly
-soft to softly
-cost to costly

Adjectives that end with an AL will end with ALLY when the suffix is added:
-final to finally
-formal to formally
-financial to financially
-general to generally

Adjectives that end with FUL will end with FULLY when the suffix is added:
-careful to carefully
-beautiful to beautifully
-peaceful to peacefully
-successful to successfully

RULE 2
When an adjective ends with E, just add LY,
giving the word an ending of ELY:
-polite to politely
-definite to definitely
-mere to merely
-complete to completely
-large to largely
-collective to collectively

When a word ends in LE, preceded by a consonant, drop the E and add Y:
-simple to simply
-terrible to terribly
-possible to possibly
-gentle to gently

RULE 3
Adjectives with more than one syllable that end with Y, change the Y to I and add LY:
-busy to busily
-easy to easily
-steady to steadily
-happy to happily

RULE 4
One syllable adjectives that end with Y, add LY to the end of the word:
-shy to shyly
-sly to slyly
-coy to coyly

The exceptions to this rule are:
-day to daily
-gay to gaily

RULE 5
Adjectives ending in IC, add ALLY to the end of the word:
-optimistic to optimistically
-music to musically
-frantic to frantically
-automatic to automatically

RULE 6
The exceptions any of these rules are:
-due to duly
-whole to wholly

Es gibt auch Substantive and verb that end with LY, but those are not suffixes added; for example:
-family
-fly
-holly
-monopoly