Some words ending in -s do not change in the plural. Common examples: barracks, crossroads, means, headquarters, series, species, Swiss.
Note that some singular uncountable nouns end in -s. These have no plurals. Examples are news, billiards, draughts (and some other names of games ending in -s), measles (and some other illnesses).
Most words ending in -ics (e.g. mathematics, physics, athletics, politics) are normally singular uncountable and have no plural use.
- Too much mathematics is usually taught in schools.
- NOT:
Too many mathematics are
Some words ending in -ics (e.g. politics, statistics) can also have plural uses.
- Politics is a complicated business.
- (BUT: What are your politics?)
- Statistics is useful in language testing.
- (BUT: The unemployment statistics are disturbing.)
Other nouns which do not change in the plural are craft ( meaning 'vehicle'), aircraft, hovercraft, spacecraft, Chinese, Japanese (and other nationality nouns ending in -ese), sheep, fish, deer, and the names of some other living creatures (especially those that are hunted or used for food).
Dozen, hundred, thousand, million, stone (= 14 pounds) and foot (=12 inches) have plurals without -s in some kinds of expressions.
Dice (used in board games) is originally the plural of die, which is not now often used in this sense; in modern English dice is generally used as both singular and plural.
Data is originally the plural of datum, which is not now used; in modern English data is used as both singular and plural.
[source: Michael Swan, Practical English Usage]
[source: Michael Swan, Practical English Usage]
