Anyone familiar with grammar and spelling knows how often standard English breaks its own rules. When words deviate from standard rules, they are considered “irregular.” Such is the case with many plural nouns. With regular nouns, the plural is formed simply by adding an “s” or “es” to the singular form. If the singular form ends in a “y,” the “y” is changed to an “i” before adding “es.” When plural forms deviate from these rules, the noun is considered irregular.
For nouns that end in “f” or “fe,” the plural is formed by changing the “f” to a “v,” before adding “es.” For example: knife becomes knives, half becomes halves. If a word ends in an “o,” such as potato or tomato, “es” is added to form the plural, as in potatoes and tomatoes. For nouns ending in the letters “us,” such as cactus and stimulus, the plural is formed by changing the “us,” to an “i,” as in cacti, and stimuli. For words ending in “is,” as in crisis or thesis, the “i” is changed to an “e” to form the plural. The plurals are therefore “crises” and theses.” Sometimes, when a noun ends in “on,” the plural is formed by changing the “on” to an “a,” such as “phenomenon” to “phenomena,” and “criterion” to “criteria.”
Reforming the Word (Or Not)
Sometimes, when forming the plural of an irregular noun, the entire spelling of the word changes. Such is the case in nouns such as child, which becomes children; person becomes people and mouse becomes mice. Occasionally, you will find an irregular noun that decides it doesn’t want to change at all. The singular and the plural are the same word. Such is the case in nouns such as deer, sheep and moose.
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