You can probably see the limitations by placing your fingers in standard position and leaving it there. You can't play the other keys because you don't have twenty hands! That's why you have to shift your finger around. For example, look at the following two measures.

How are you going to play the higher B and C? Here's how you do it. Read this carefully because it's very important. When you get to the E, leave your middle finger on the E the move your thumb to the G. Press the G with your thumb. Then, with the middle finger, play B and use the ring finger for the C. Once you get back down to the G, you must shift your hand back down to standard position. To do this, leave your thumb on the G and move your middle finger down to the E. You're done! Go on to the next page to continue this lesson.