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 region adjacent to the first key button of the keypad, in response to the first signal, the first sub key button partially overlapping a second key button neighboring to the first key button, 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 same region adjacent to the first key button of the keypad, in response to the second signal, the second sub key button partially overlapping the same second key button neighboring to the first key button, 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.