Singular and Plural nouns in English

Most countable nouns have both a singular and a plural form, the singular for one thing and the plural for two or more.
REGULAR PLURALS
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

IRREGULAR PLURALS

1. With seven nouns you change the vowel: man / men,  woman  / women,  foot / feet,  goose / geese,  mouse / mice,  tooth / teeth, louse / lice
2. 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.)
3. With three nouns you add -en: ox / oxen, child / children, brother / brethren (only in the religious sense)
4. 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)
5. A few nouns have no plural ending, but you can still use them in a singular or plural way: they include the names of some animals (such as sheep, deer, cod), certain nationalities (such as Portuguese, Swiss), some nouns expressing quantity (such as ton, p (=”pence)),” and a few others (such as aircraft, crossroads, kennels, offspring).

• 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)

EXERCISES

• English at Home- exercise