All statements are sentences but not all sentences are statements. Statements are sentences which are true or false. Sentences which are neither true nor false are not statements. For example, threats, commands, promises, questions and requests are not statements. If I say "Please, close the door," I am not saying anything that is either true or false. I have not made a statement. My request may be reasonable or unreasonable, but it is neither true nor false. Another example is promising. I may make a promise fully intending to keep it or I might make the promise without intending to do as I promised. But in the first case the promise is sincere, not true. In the second case, the promise is insincere, not literally false. Finally, suppose I advised you to take all your money out of the stock market and buy lottery tickets instead. I would then be giving you bad or foolish advice. I would not be giving you false advice.
Because the goal of critical thinking is to believe what is true and avoid what is false, critical thinking is mostly about statements. Arguments, as we shall see shortly, are composed entirely of statements. As we said above, statements are sentences that are either true or false. Many of the statements we are concerned with are controversial or just uncertain. People do not have to agree that a statement is true in order for it to be a statement. Nor does anyone have to know whether or not the sentence is true for it to be a statement. It just has to be either true or false.