This Purdue Online Writing Lab article explains the three uses of the apostrophe.

In this second unit of an introductory Spanish course, students learn how to use days of the week in conversation and in writing, how to change nouns between singular and plural, how to recite a children's song, how to identify and model...

Resource explains the three uses of the apostrophe and provides examples. Requires Adobe Reader. [PDF]

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.

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.

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...