Учебное пособие: Консультации по английской грамматике В помощь учителю иностранного языка

If a noun ends in -o, -es is added in the plural number: tomatoes, potatoes, vetoes. Only -s is added if a noun ends in a vowel + o: bamboos, studios, zoos; in proper names: Romeos, Eskimos, Philipinos; in abbreviations: kilos, photos, pros (professionals); also: stereos, discos, videos, pianos, solos. Some other nouns take -s or -es: cargo, banjo, halo.

If a noun ends in -y, it is changed into -i- and -es is added: armies, duties, stories. If a vowel precedes -y, just -s is added: boys, keys, plays.

Some old English plural forms are still used:

man — men goose — geese child — children

woman — women louse — lice ox — oxen

foot — feet mouse — mice brother — brethren

tooth — teeth

Some English nouns keep foreign plurals, sometimes English and foreign plurals are used side by side:

alumna — alumnae

alumnus — alumni

formula — formulae, formulas

index — indices, indexes

crisis — crises

criterion — criteria

phenomenon — phenomena

datum — data

nucleus — nuclei

syllabus — syllabi, syllabuses

Compound nouns have three ways of spelling: two components are separated by a hyphen, written in one word or written separately. To form the plural of compound words spelled with a hyphen -s is added to the principal word:

daughters-in-law passersby

editors-in-chief girlfriends

lookers-on watchmakers

When compound singular nouns are spelled as one solid word, they are pluralized by adding -s to the last word:

airships bookcases

baseballs bywords

beefsteaks churchgoers

The plural of compound words with vowel-changing words in them will be:

airmen

mailmen