The controlling statement merely introduces the topic of the composition. The controlling statement may be introduced in three different ways depending on whether you are asked to write a narrative, a descriptive, an expository, or an argumentative composition. The central idea statement simply introduces the topic, usually by means of a simple sentence.
Cheeseburgers are nutritious.
My uncle is the most unforgettable character I have ever met. This way of introducing the topic works very well for narrative and descriptive compositions. The statement of purpose always contains, or starts with, an infinitive. I am going to offer an alternative explanation for the cause of the American Civil War. I am going to describe my scuba-diving trip to Cancun.