Collins Cobuild English Grammar (206 page)

R138
  Sometimes the formation is slightly different.

With adjectives ending in
le
, you replace the
le
with
ly
.

suitable		suitably
terrible		terribly
gentle		gently

Note that
whole
has the related adverb
wholly
.

R139
  With adjectives ending in
y
, you replace the
y
with
ily
.

easy		easily
satisfactory		satisfactorily

Note that one-syllable adjectives ending in
y
usually have
ly
added, in the normal way.

wry		wryly
shy		shyly

Note that the adverb related to
dry
can be spelled
drily
or
dryly
.

R140
  With adjectives ending in
ic
, you add
ally
.

automatic		automatically
tragic		tragically

Note that
public
has the related adverb
publicly
.

R141
  With a few adjectives ending in
e
(not
le
), you replace the
e
with
ly
.

due		duly
true		truly
undue		unduly
eerie		eerily