Plural Forms of Nouns

1.    To make most nouns plural, add –s.

(a)	Singular	Plural
	One dog	Two dogs
	One book	Three books
	A cake	Some cakes

2.   Add -es to nouns ending in -sh, -ch, -ss, and -x.

(b)	Singular	Plural
	One dish	Two dishes
	One match	Three matches
	One class	Two classes
	A box	Some boxes

3.    If a noun ends in a consonant + -y, change the -y to i  and add -es (c).

If -y is preceded by a vowel, add only -s, as in (d).

(c)	Singular	Plural
	A baby	Two babies
	One city	Three cities
	one try	Some tries
(d)	One toy	Two toys
	One key	Three keys
	A tray	Some trays

4.       If a noun ends in -fe or -f, change the ending to -ves.

(Exceptions: beliefs, chief, roof, cuffs, cliffs.)

(e)	Singular	Plural
	One knife	Two knives
	One shelf	Some shelves
	A wife	Two wives

5.       The plural form of nouns that end in -o is sometimes -oes and sometimes -os.

-oes: tomatoes, potatoes, heroes, echoes

-os: zoos, radios, studios, pianos, solos, sopranos, photos, autos, videos

-oes or -os: zeroes/zeros; volcanoes/volcanos, tornadoes/tornados, mosquitoes/mosquitos.

(f)	Singular	Plural
	One tomato	Two tomatoes
	One zoo	Two zoos
	One zero	Two zeroes/zeros

6.       Some nouns have irregular plural forms.

(Note: The singular form of people can be person, woman, man, child. For example, one man and one child = two people.)

(g)	Singular	Plural
	One mouse	Two mice
	One louse	Two lice
	One tooth	Three teeth
	One foot	Two feet
	One goose	Some geese
	An ox	Two oxen
	One child	Two children
	One man	A few men
	One woman	Two women
	---------------	Two people

7.       The plural form of some nouns is the same as the singular form.

(h)	Singular	Plural
	One deer	Two deer
	One fish	Three fish
	A means	Two means
	One sheep	Two sheep
	One series	Three series
	An offspring	Some offspring
	A species	Some species

8.       Some nouns that English has borrowed from other languages have foreign plurals.

(i)	-us → i
	Singular	Plural
	One alumnus	Two alumni
	One cactus	Two cacti
	One focus	Two foci/focuses
	One fungus	Two fungi/funguses
	One nucleus	Two nuclei
	One radius	Some radii
	One stimulus	Two stimuli
(j)	-is → es
	Singular	Plural
	One axis	Two axes
	One analysis	Two analyses
	One basis	Two bases
	One crisis	Two crises
	One diagnosis	Two diagnoses
	One ellipsis	Some ellipses
	A hypothesis	Two hypotheses
	An oasis	Two oases
	One paralysis	Two paralyses
	A parenthesis	Two parentheses
	One synopsis	Three synopses
	One thesis	Two theses
(k)	-ix → ices
	Singular	Plural
	One appendix	Two appendices
	One index	Two indices/indexes
	One matrix	Two matrices/matrixes
(l)	-eau → eaux
	Singular	Plural
	One beau	Two beaux
	One bureau	Two bureaux/bureaus
	One tableau	Two tableaux/tableaus
(m)	-um and others → a
	Singular	Plural
	One bacterium	Two bacteria
	A curriculum	Two curricula
	One datum	Two data
	One medium	Two media
	A memorandum	Two memoranda
	A phenomenon	Some phenomena
	One stratum	Three strata
	One corpus	Two corpora
	One criterion	Three criteria
	One genus	Two genera
(n)	-a → ae
	Singular	Plural
	One antenna	Two antennae/antennas
	One formula	Two formulae/formulas
	One vertebra	Two vertebrae
	One vita	Two vitae

References:

Fundamentals of English Grammar (3^rd Ed) by Betty Azar

http://english-zone.com