Collins Cobuild English Grammar (195 page)

R57
    With verbs which end in
sh
,
ch
,
ss
,
x
,
zz
, or
o
,
es
rather than
s
is added to the base form of the verb. The
es
is pronounced
/
I
z/
when it is added to a consonant sound, and pronounced
/z/
when it is added to a vowel sound.

diminish		diminishes
reach		reaches
pass		passes
mix		mixes
buzz		buzzes
echo		echoes

R58
     With one-syllable verbs that end in a single
s
, you usually add
ses
. Forms with a single
s
are more common in American English.

bus		busses		buses
gas		gasses		gases

R59
    Most verbs have
-ing
participles formed by adding
ing
to the base form, and past forms formed by adding
ed
to the base form.

paint		painting		painted
rest		resting		rested