Lumen Learning
Lumen: Writing Skills: Possessives
This lesson focuses on recognizing and demonstrating the standard use of apostrophes when using both singular and plural. It also provides a practice exercise.
Online Writing Lab at Purdue University
Purdue University Owl: The Apostrophe
This site focuses on the three uses for apostrophes: forming possessives of nouns, forming plurals of lowercase letters, and showing omission of letters and provides examples of each.