Word structure: Inflection

Inflection is the process by which a single word takes different forms. For example, if we have the noun cat, we can add a plural ending to it to create cats. This is known as inflecting a noun and the ending we add is called a suffix.

What are the plural forms of the following nouns?

  • girl
  • boy
  • class
  • quiz
  • curry

Can you see a pattern in how these plurals are formed? All these words take the regular plural ending -s or -es, though in some cases there is also an adjustment in the spelling:

  • girlgirls
  • boyboys
  • classclasses
  • quizquizzes (final consonant is doubled)
  • curry curries (y is replaced by i)

What about the following nouns? What are their plural forms?

  • knife
  • mouse
  • tooth
  • sheep
  • formula

You will have noticed that it’s not always as straightforward as adding an -s or -es suffix. Some words have special (‘irregular’) patterns for forming the plural:

  • knifeknives: Here we find a change of the final consonant.
  • mousemice: This shows a change of vowel. (But what about a computer mouse? Here people vary. Would you say mouses or mice?)
  • toothteeth: This too has a vowel change.
  • sheepsheep: Here there is no change at all.
  • formulaformulae: Here the final -a changes to -ae.

The last item (formulae) is an example of a foreign plural: a noun borrowed from a foreign language (Latin, in this case) which changes in the plural according to the original language. Sometimes a regular plural is used instead, for example formulas instead of formulae.

Most nouns have singular and plural forms, and we use the different forms in different grammatical patterns. Here are some examples:

  • this book, a book (not this books, a books)
  • these books, three books (not these book, three book)
  • This book is ... (not This book are ...) (Complete the sentence with e.g. heavy, long, boring.)
  • These books are ... (not These books is ...)

The different patterns work with other nouns as well, e.g. this knife, these knives. Try them out with some other nouns, such as guitar, tooth, mistake.

Noun plurals are just one example of inflection. Verbs also have different inflectional forms, as shown in the following example:

  • The man was turning the TV channel over. When he had turned it over he started watching a programme about a man who turns into a wolf at full moon.

Here we see various forms of the verb turn, formed by adding different suffixes (-ing, -ed, -s). Verb inflections help to express when a situation occurs and how it unfolds. The different verb forms are explained further in the resources on verbs.

Many adjectives also take different inflectional forms. An adjective such as small has a comparative form which adds an -er suffix and a superlative which adds an -est. The plain form has no added inflection (My room is small). The comparative and superlative forms are used in comparisons:

  • My room is smaller than my brother’s. (comparative)
  • My room is the smallest room in the house. (superlative)


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