Nouns can be either singular (desk) or plural (desks). Nouns can be either count nouns, which names things that can be counted and have plurals (chairs), or noncount nouns, which name things that cannot be counted and do not have plurals (furniture).

TIP: Putting the word much in front of a noun will help you to determine whether it is count or noncount.

Much homework
(much makes sense, so homework is a noncount noun)

Much pencils
(much does not make sense here, so pencil is a count noun)

For more information on singulars and plurals, see Book I, Ch. 3; Book II, Ch. 16; Book III, Ch. 14.