When you use "by," it means that something will happen no later than a specified time. For example, if a teacher gives me a project for class, the teacher may say that I need to have that project finished "by" September 3. In this case, I can finish the project any time before September 3, but it must be finished no later than September 3. When you use "until," it means that something must happen before something else can happen. For example, my mother may say that I cannot go to my friend's house until I clean my room. In this case, I must clean my room before I can go to my friend's house. I hope I was able to help. :)