The most common reason alcoholic beverages are used in a recipe is to impart flavor. Extracts with the most intense flavors are alcohol-based, particularly vanilla.
In some recipes, alcohol is used to achieve a desired chemical reaction in a dish. Beer contains yeast that leavens breads and batters. Sometimes, alcohol is used in marinades to help break down tough fibers in meats. Other dishes use alcoholic content to provide entertainment, such as flaming dishes. In these cases, it is important to choose a substitute that will achieve a similar effect.
Wine was originally added to fondue to lower the boiling point of the cheese to prevent curdling. You may not be able to find an adequate substitute in that instance. In the case of leavened goods, you can try club soda to replace beer, but the results will be subtly different. You might also consider a different recipe that uses another leavener such as yeast, baking powder or baking soda. For marinades, acidic fruits will usually do the trick. For flambees and flamed dishes, you're out of luck and you should plan to use a different recipe. For flavoring alone, you will often have a number of substitution options.
see previous post -- Part One
Stay tuned and Keep Coming Back for Part Three.