The English language has thousands of words that appear in our day to day usage. We often want to use the plural of some words so as to convey certain messages. For instance CARS is the plural of CAR. Adding S to a word is the most common convention of arriving at plurals of words. However, this does not always work! There are many other words you might have come across such as ENEMY-ENEMIES, CATTLE-CATTLE and ALUMNUS-ALUMNI.
As you have learnt in earlier quizzes, there are some conventions which can be followed in English and forming plurals too follows conventions. Apart from the most common convention of adding S to a word to form its plural there are others that merit attention:
Some words ending in X or S become plural by adding ES at the end of the word, such as BOX-BOXES or HOUSE-HOUSES.