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.

This site gives a very clear explanation of a gerund and its noun qualities. This resource explains how to use the possessive case with a gerund. A quiz is provided to check understanding of this concept.

Discusses the two main functions of apostrophes: contractions and forming possessives.