Sorry about the length of the following - hope it's helpful. The differences between these two words are rather subtle, and it is probably best to consider their use as verbs and nouns separately. As a verb "to effect" means "to cause", but "to affect" means "to change". Thus, "this diet will effect weight loss" means that it actually brings about weight loss; but "this diet may affect your health" means that the change could be for the better or the worse. "To affect" can also mean "to pretend", as in "he affected a French accent". As a noun, "effect" is fairly common and straightforward - it means a change in something, such as "rain always has a depressing effect on me". "Affect" can also be used as a noun, though it is less common and generally only used in a psychological context - it means feeling or emotion, as in "the patient showed little affect when discussing his father's death".