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Khan Academy
Khan Academy: Introduction to the Possessive
Learn to use apostrophes to indicate possession, or ownership -- and when not to use them.
Online Writing Lab at Purdue University
Purdue University Owl: Possessive Pronoun and Apostrophe
This Purdue Online Writing Lab article explains the three uses of the apostrophe.
Other
Bellevue Community College: Apostrophes [Pdf]
Resource explains the three uses of the apostrophe and provides examples. Requires Adobe Reader. [PDF]
SUNY Empire State College
Empire State College: Apostrophes
This resource looks at the three main uses of apostrophes: to take the place of deleted letters; to show possession; and to show plurals of letters, numbers, symbols, abbreviations and words used as words.
English Plus+
Grammar Slammer's Apostrophes Showing Possession
This site discusses the correct and incorrect usage of apostrophes to show possession and examples are provided. Note that links are provided at the bottom of the page for other specific uses of the apostrophe.
English Club
English Club: Learn English: Grammar: Verbs: Subject Verb Agreement
An explanation and examples of subject-verb agreement including the use of compound subjects, midsentence words, singular/plural nouns, inverted subjects, collective nouns, indefinite pronouns, and fractions. Links to additional...