Grammatical case refers to morphological inflections (changes in the form) of a noun, adjective, or pronoun depending on that word’s function as subject, object, object of a preposition, etc. in a sentence. While earlier forms of English like Old English and sometimes even Middle English had clear morphological distinctions between accusative and dative forms, Modern English does not.

In Modern English, only the personal pronouns and who/whom/whose inflect for case. (This deliberately disregards the genitive construction -’(s), which is a questionable example as it’s not a core case and it’s disputed whether it should be analyzed as an inflection or as a clitic when it appears on a single-word noun phrase.)

The two core cases: subject and object

English pronouns have only two core-case forms at most:

• Subject case: I/(thou)/he/she/they went to the store. (Thou is archaic.)
• Object case: She looked at me/(thee)/him/her/them. (Thee is archaic.)

The personal pronouns you and it use the same form no matter whether they are subjects or objects. The interrogative or relative pronoun who uses only one form for both subject and object in colloquial speech, but the form whom may be used for an object in formal speech or writing.

The subjective form may also be called the nominative, and the objective form may be called the accusative. This particular use of accusative for the object case is somewhat misleading, as the Modern English object case now covers both the accusative (“direct object”) and dative (“indirect object”) functions of the pronoun.

The possessive or genitive case

The personal pronouns also all have special possessive forms (also called genitive).

• One set of possessive pronouns are used as determiners in a noun phrase, and are sometimes called possessive adjectives or possessive determiners: my, (thy), your, his, her, its, their. (Thy is archaic.)
• Another set of possessive pronouns are used substantively as an independent noun phrase in their own right, whether as a subject or an object: mine, (thine), yours, his, hers, (its), theirs. (Thy is archaic, and its is rarely used independently.)

There is also a special interrogative or relative possessive pronoun, whose.

Pronunciation and spelling of possessive nouns

The possessive of nouns or other noun phrases is formed in writing by adding the written suffix or clitic ‑’s, such as in “this cat’s tail” or “the Queen of England’s hat”. For more information on this clitic construction where the possessive is added to the entire noun phrase, see the questions and tag info for saxon-genitive.

This possessive ‑’s may be pronounced /s/ as it cat’s, /z/ as in kid’s, or /əz/ as in church’s. The pronunciation of the possessive (regular) singular noun matches that of the regular plural noun or singular verb, so kids as a plural noun or as a singular verb is pronounced just like the singular possessive noun kid’s with an added /z/ sound. It’s also the same as kids’ in the possessive plural.

The rule is that the written ‑’s form is used to form the possessive whenever another sound is added to the base form of the noun in speech. That’s because here the spelling is intended to reflect the possessive form’s distinctive pronunciation.

However, if the noun is a plural noun that already ends in a pronounced s like cats or farmers, then because the pronunciation of the word does not change in the possessive, only a bare ‑’ is used in writing to show that the word is possessive, such as as in “those cats’ tails” or “these farmers’ cows”.

Plurals that do not already end in an s sound are pronounced with an extra one when used possessively, and so we write those with ‑’s: fish’s, sheep’s, men’s, women’s, children’s, oxen’s, bacteria’s, alumni’s, phenomena’s. (The actual rule is more complicated that this, which is why we write and pronounce possessive plurals mice’s and geese’s with two syllables even though their plural nouns already ended in an s sound.)

The bare apostrophe is used in a few cases at the end of a singular noun that ends in an unstressed /iz/ sound like this species’ cousin or this series’ finale, or in Greek names like Socrates’ friends or Achilles’ heel. This is because there is no additional s sound added to those words when they’re used possessively, and so we do not write an extra s letter.

You may also sometimes see written forms such as for goodness’ sake without an extra s because there is no extra syllable added there when said.

Although in older forms of writing, English possessives did not always strictly reflect their pronunciation, most modern style guides (excepting those used in newspaper and magazine publishing to shorten up lines of text to reduce print costs) normally recommend doing so. Therefore one writes James’s to represent a possessive pronunciation with three syllables, but just James’ when writing a possessive pronunciation with only two syllables when they don’t add a new sound to make it possessive. This varies depending on the speaker’s own spoken usage.