Apostrophe Basics: 's and s'

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