The noun which represents one person, place or thing is known as singular noun.
e.g: man, cat, flower etc.

The noun which stands for more than one person, place or thing is known as plural noun.
e.g: men, cats, flowers etc.

a. In many cases only "s" is added to the singular noun to change it into plural.

b. If singular noun ends in S, X, Ch, Ox plural is formed by adding "es"

c. In case a noun ends in 'f' or 'fe' is dropped and 'ves' is added.

d. Some nouns ending in 'f' or 'fe' can be made plural by adding 's' only.

e. If singular noun ends in 'y' it forms its plural by dropping 'y' and adding 'ies'.

f. Some words are always singular.

g. If a noun ends in 'o' es is sometimes added, especially in the following cases.