Larry: Try not to get frustrated at this point. Languages are arts, not sciences, and therefore have guidelines rather than rules, and lots more exceptions to the guidelines.
When you are first learning (as I assume you are since you are on Lesson 1.4) you run into lots of confusing things. Later on another lesson will explain them further, or your curiosity will compel you to do some independent research, and you will have a eureka moment.
Early in my study, I experienced many of these frustrations. I am certain that other have too. I wondered at the time why Amy did not give me a little heads up on these things, but maybe it would be a case of too much information too soon. Or maybe they hope you will find out some things on your own and the hard earned knowledge will be forever cemented in your head.
When you find something you don't understand, just post it here. Someone will help you over the hurdle.