a / an / plural nouns

In English, the plural is usually formed with the addition of -s (e.g., one cat, two cats; one chair, two chairs) or -es (e.g., one bush, two bushes; one itch, two itches). Generally, -s is added to all nouns that end in a voiceless consonant, vowels, or voiced non-sibilants, whereas -es is added for nouns ending in a sibilant sound. Nouns that end in e are a noted exception; though e may form a sibilant sound, -s is used (e.g.,. one tree, two trees; one bee, two bees).

Some plural forms require more noticeable changes in word structure. Most words ending in a y preceded by a consonant are pluralised with ies (e.g., one lady, two ladies; one cherry, two cherries). Some words ending in f are pluralised with -ves (e.g., one leaf; two leaves; exception: one roof; two roofs). Words ending in x are often pluralised with -ces (e.g., one matrix, two matrices; one index, two indices). Words ending in us often replace the us with -i (e.g., one cactus, two cacti; one fungus, two fungi). A subset of words ending in um or on are pluralised by replacing with -a (e.g., one forum, two fora; one criterion, two criteria).

Other variations occur where vowels and consonants change in the middle structure of the word. Such examples are, (1 die; 2 dice), (1 goose; 2 geese) and, (1 mouse; 2 mice).

A small class of words have identical singular and plural forms: e.g., one sheep, two sheep; one aircraft, two aircraft.



Resources from :

wikipedia.org
Headway Elementary Book