1. A digital television comprising: a display unit configured to display a keypad including a plurality of key buttons which are assigned with different alphabet characters, respectively, and a text window; and a controller configured to: receive a first signal selecting a first key button of the plurality of key buttons, display a first alphabet character assigned with the first key button in the text window, and display a first sub key button assigned with a second alphabet character on a first region adjacent to the first key button of the keypad, in response to the first signal, and receive a second signal selecting the first sub key button, and display the second alphabet character next to the first alphabet character in the text window, and display a second sub key button assigned with a third alphabet character on the first region adjacent to the first key button of the keypad, in response to the second signal, wherein the second alphabet character is predicted as a next character of the first alphabet character, and wherein the third alphabet character is predicted as a next character of the first and second alphabet characters.