Nouns that end in "f" are irregular and cannot be made plural by adding "s" or "es." Instead, replace "f" with "v" and add "es." For example, "half" becomes "halves," "leaf" becomes "leaves" and "shelf" becomes "shelves."
Nouns that end in "fe" are irregular. To make one of these nouns plural, change "f" to "v," keep "e" and add "s" to the end of the word. For example, "knife" becomes "knives," "life" becomes "lives" and "wife" becomes "wives."
Nouns that end in a consonant and then "y" are irregular. Change "y" to "i" and add "es" to the end of the word. For example, a "poppy" becomes two "poppies," a "puppy" becomes two "puppies" and a "spy" becomes two "spies."
Nouns that end in "o," such as "tomato," are irregular. Add "es" to the end of the word to make it plural, as in "potatoes" and "tomatoes."
An irregular noun ending in "us" will have a different ending in its plural form. Replace "us" with "i." For example, "nucleus" becomes "nuclei" and "focus" becomes "foci."
For an irregular noun ending with "is," change the ending to "es." For example, "analysis" becomes "analyses," "crisis" becomes "crises" and "thesis" becomes "theses."
A noun ending in "on" is irregular; change the ending from "on" to "a" to create the plural. For example, the plural of "phenomenon" is "phenomena," and the plural of "criterion" is "criteria."
The spellings of some plural nouns do not change from their singular forms. For example, "fish" can be either the singular or the plural form, as can "sheep."
Some irregular nouns do not follow set patterns to be made plural. Instead, the spellings are learned separately for both singular and plural forms. For example, "child" becomes "children," a changed ending; "person" becomes "people," a word change; and "tooth" becomes "teeth," a vowel change.