When do I single-click and when do I double click?
Don't double-click everything. Windows 101: Double-clicking is how you open items in Windows. It's not how you open links in your Web browser, click buttons in dialog boxes, or do pretty much anything else--and if you reflexively double-click, you might accidentally zip right past something important or submit a form twice. If you don't need this reminder yourself, chances are you know someone who does.
If you move the curser over an item and there is a change to the item or the curser, then you only need to single click.