The regular way of changing a noun from singular to plural is to add -s at the end:
- cat / cats, book / books, house / houses
For nouns ending in consonant + -y, you drop the -y and add -ies to form the plural:
- family / families
For nouns ending in -o, you add -es to form the plural.
- tomato / tomatoes, potato / potatoes
- With seven nouns you change the vowel: man / men, woman / women, foot / feet, goose / geese, mouse / mice, tooth / teeth, louse / lice
- With a few nouns you change the final -f to -ve before adding the -s ending: knife / knives, leaf / leaves, wife / wives, half / halves (Some nouns in this group have a regular plural as well: scarfs and scarves, hoofs and hooves. Both possibilities are correct.)
- With three nouns you add -en: ox / oxen, child / children, brother / brethren (only in the religious sense)
- A few nouns which have been borrowed from foreign languages have an irregular plural. They include: stimulus / stimuli, crisis / crises, criterion / criteria, phenomenon / phenomena. Often these nouns have two plurals: they have developed a regular plural but have also kept their original irregular one. In these cases, the regular form is more informal and popular; the irregular form tends to be used by specialists.
- There are no certain formulas for success. (informal)
- We have to learn all the relevant chemical formulae. (specialist)
- The sheep was making a noise. The sheep were making a noise.
NOUNS WHICH ARE ONLY SINGULAR OR PLURAL
Several nouns are used only in the singular, particularly proper names (Smith, Monday), uncountable names (music, geography, advice) and some exceptions ending in -s (physics, billiards)
Some other nouns are used only in the plural, such as, nouns refering to objects consisting of two parts (scissors, trousers), nouns ending in -s and only used in the plural (stairs, thanks), and nouns expressing the idea of a group of people or animasl (police, folk, cattle, poultry)