The majority of nouns in English spell their plural by simply adding a final -s. Nouns that are noncount or abstract (e.g., cheese, sugar, honesty, intelligence) generally take a singular verb, but in some instances can be plural, in which case they follow the rules for plural based on their spelling. Also, there are some categories of words which are only plural, even though their spelling does not reflect this. They are included in a list at the end of this page. For irregular count nouns and nouns that have been borrowed from other languages, the rules are as follows:

Variations of the final -s rule:
Nouns that end with -s, -z, -x, -sh, -ch 
Add -es glass/glasses, buzz/buzzes, box/boxes, bush/bushes, switch/switches 
 
Nouns that end in -o 
Add -es potato/potatoes, echo/echoes, hero/heroes exceptions: studio/studios, piano/pianos, kangaroo/kangaroos, zoo/zoos either: buffalo/buffalo(e)s, cargo/cargo(e)s, motto/motto(e)s,  volcano/volcano(e)s

Nouns that end in a consonant + -y 
Change -y to -i and add -es baby/babies, spy/spies, poppy/poppies

Nouns that end in -f, or -fe 
Change the -f to -v and add -es

shelf/shelves, wolf/wolves, knife/knives, wife/wives

Nouns adopted from other languages:
Singular ends in -is 
Plural ends in -es analysis/analyses, basis/bases

Singular ends in -um 
Plural ends in -a datum/data, curriculum/curricula

Singular ends in -on 
Plural ends in -a criterion/criteria, phenomenon/phenomena

Singular ends in -a 
Plural ends in -ae formula/formulae, antenna/antennae

Singular ends in -ex or -ix 
Plural ends in -ices appendix/appendices, index/indices

Singular ends in -us 
Plural ends in -i focus/foci, stimulus/stimuli

Singular ends in -us 
Plural ends in -a corpus/corpora, genus/genera

Singular ends in -eau 
Plural ends in -eaux bureau/bureaux, beau/beaux

Nouns that have only a plural form and so take a plural verb
Things that come in pairs 
Tools: glasses, scissors, binoculars, forceps, tongs, tweezers Clothes: jeans, pants, pajamas, shorts, trousers

Nouns that end in -s but have no singular (aggregate nouns) 
accommodations, amends, archives, arms (weapons), bowels, intestines,  brains (intellect), clothes, communications, congratulations, contents,  stairs, thanks, goods

Nouns that are plural but do not end in -s 
people, police, cattle, people