Gerunds versus Infinitives

The University of Victoria does a good job of summarizing when to use gerunds (-ing ending) versus infinitives (formed with "to"):

Gerunds	Infinitives
Gerunds are often used when actions are real, concrete, or completed. For example: I stopped smoking. (The smoking was real and happened until I stopped.)	Infinitives are often used when actions are unreal, abstract, or future. For example: I stopped to smoke. (I was doing something else, and I stopped; the smoking had not happened yet.)

The following websites provide additional explanations that will help you differentiate between when you should use a gerund versus an infinitive.  They also provide exercises to practice.

Website	Page Title	Comments
University of Victoria	Gerunds & Infinitives	Explanation + Exercises
English Page	Gerunds and Infinitives	Explanation + Exercises