Here are some differences: “Prepare” is usually something a person does to a thing, to make the thing complete/ready. Example: “I need to prepare the food for dinner.” “You need to prepare the surface of the wood by sanding it before you paint it.” In contrast, “prepare for” usually is used when talking about getting YOURSELF ready for something. Example: “I need to prepare for the test.” As to “forget” and “forget about,” I think that they are interchangeable, sometimes with slight differences in grammar as shown in the following examples: “Don’t forget to lock the door” = “Don’t forget about locking the door.” “She forgot that she had an appointment today” == “She forgot about her appointment today.” “I will never forget your kindness to me” == “I will never forget about your kindness to me.”