Collins Cobuild English Grammar (197 page)

Note that the past form of such verbs is regular, following the pattern in
R60
.

R63
    To form the past form of a verb that ends in a consonant letter followed by
y
, you substitute
ied
for
y
.

cry

cried

Note that the
-ing
participle of such verbs is regular, following the pattern in
R59
.

R64
    If the base form of a verb has one syllable and ends with a single vowel letter followed by a consonant letter, you double the final consonant letter before adding
ing
to form the
-ing
participle or
ed
to form the past form.

dip		dipping		dipped
trot		trotting		trotted

Note that this does not apply if the final consonant letter is
w
,
x
, or
y
.

row		rowing		rowed
box		boxing		boxed
play		playing		played

R65
    The final consonant letter of some two-syllable verbs is also doubled. This happens when the second syllable ends in a single vowel letter followed by a consonant letter, and is stressed.

refer		referring		referred
equip		equipping		equipped