How to Tell If a Guy Likes You

Si-woo feels something is wrong with Do Ha-na and him. While he does not know what to do, his brother tells him to go and talk to her. He also mentions she wouldn't know if Si-woo won't talk. So Si-woo decides to tell his side of the story of how he started liking Do Ha Na to her. As he runs to see her, he sees Do Ha-na with someone else.