Apostrophe Basics: 's and s' — English Grammar Exercises

Apostrophe Basics: When to Write 's and When to Write s'

Apostrophe errors are the most visible possessive mistakes in English writing, and they follow predictable patterns. Research from the Cambridge Learner Corpus identifies three recurring error types at B1–B2 level: adding an apostrophe to a regular plural (students' correctly vs students's incorrectly), omitting the 's from irregular plurals (writing childrens' instead of children's), and applying the plural apostrophe rule to singular nouns ending in -s (writing boss' instead of boss's). Together, these account for the majority of possessive errors in B1–B2 learner writing.

The Two-Step Rule

To place the apostrophe correctly, follow two steps: (1) form the possessive base — is it singular or plural? (2) check whether that form ends in -s. If yes, add only an apostrophe. If no, add 's.

• Singular (any ending): sister → sister's, boss → boss's, James → James's
• Regular plural (ends in -s): students → students', parents → parents', bosses → bosses'
• Irregular plural (no -s): children → children's, women → women's, people → people's

Singular Nouns Ending in -s

A common mistake is to treat singular nouns ending in -s like plural possessives. They are not. A singular noun — regardless of its final letter — always takes 's: boss's, class's, bus's. The only exceptions are some proper names in classical contexts (Jesus', Socrates'), where style guides differ.

Common Mistakes

Frequently Asked Questions

When do you add 's and when do you add just an apostrophe (s')?

The rule depends on whether the noun is singular or plural and how the plural is formed. Singular nouns — including those ending in -s — add 's: the boss's office, James's car. Regular plural nouns already ending in -s add only an apostrophe after the -s: the students' essays, my parents' house. Irregular plural nouns that do not end in -s add 's: children's playground, women's changing room, people's opinions. The key question is: what is the base plural form? If it ends in -s, add only an apostrophe. If it doesn't, add 's.

When do you use 's and when do you use an of-phrase?

's is the natural choice for people, animals, organisations, institutions, and time/distance/value expressions: the director's office, the government's decision, a week's holiday. The of-phrase is more natural with inanimate objects and physical parts of things: the roof of the house, the beginning of the film, the colour of the sky. For parts of objects, English also frequently uses compound nouns with no apostrophe: table leg (not table's leg), car door handle (not car's door handle). The rule of thumb: if you can replace it with a person's name, use 's. If the owner is an inanimate object, prefer of or a compound noun.

What is joint possession and how do you show it with apostrophes?

Joint possession means two or more people share a single item. In that case, only the last name takes 's: Tom and Sarah's wedding (one shared wedding). Separate possession means each person has their own. In that case, every name takes 's: Mark's and Lisa's offices (different offices). The grammar follows the logic: if one apostrophe covers both people, it must go on the final name in the pair. If each person owns their own item independently, each name needs its own possessive marker.

What is the double genitive and why is 'a friend of Sarah's' correct?

The double genitive (also called the 'of-possessive') is the construction: a/an + noun + of + possessive. 'A friend of Sarah's' means one of Sarah's friends — the possessive 's is required because the sentence implies Sarah has multiple friends and you are referring to one of them. Without the 's, the meaning changes: 'a friend of Sarah' would suggest the friend is somehow about Sarah or depicts her. The pattern extends to pronouns (a friend of mine, a colleague of yours) and to plural names (a neighbour of the Smiths'). The double genitive is obligatory when the indefinite article a/an precedes the noun.