Some nouns have identical singular and plural forms. Examples are: sheep, swine, deer, cod, trout, salmon, aircraft, spacecraft, series and species.

• The shepherd took his sheep to the forest.
• NASA plans to send two spacecraft to the Jupiter.

The following nouns have no plural forms when used after numerals: pair, dozen, score, gross, hundred, thousand etc.

• We bought two dozen mangoes. (NOT We bought two dozens mangoes.)
• He weighs about ten stone.
• Two hundred students attended the seminar. (NOT Two hundreds students attended the seminar.)

The following nouns are used only in the plural. Examples are: bellows, scissors, tongs, pincers, spectacles, drawers, trousers, shorts, breeches etc.
The following nouns were originally singular but are now generally used in the plural. Examples are: alms, riches, eaves

Nouns ending in –s

The names of some subjects ending in –s look plural but are in fact singular. Examples are: mathematics, physics, electronics etc.
The names of some common diseases and games are also used in the singular. Examples are: measles, mumps, rickets, billiards etc.
Certain collective nouns look singular but are always used in the plural. Examples are: poultry, cattle, people, gentry etc.

Related posts:

1. Formation of Plural Nouns
2. Countable and Uncountable Nouns
3. Noun: Number
4. Collective Nouns and Abstract Nouns
5. Proper Nouns and Common Nouns
6. Noun: Gender