While a well-written script, expert direction and convincing actors help to win over drama lovers, the final ingredient for a show's success is an excellent soundtrack.

Next time you watch a drama, notice the moment when you become hooked on the storyline. It's usually about when you start humming along with the OST and you may keep humming that song long after the episode ends.

Last month's "Gu Family Book" won the best drama viewer ratings and part of that success has to be credited to its haunting soundtrack.

The OST for this drama about a half-human half-mythical creature who yearns to become human features four great songs.

The first and most easily identifiable is the song "My Eden" delivered by operatic pop singer Yisabel. The song sums up the Eden that Gu Wol Ryung, a mythical mountain spirit, forsakes for love. Yisabel, a finalist for the Metropolitan Opera, performs the hopeful yet sad song that is about what might-but will not-be.

A second song on the soundtrack is "Love Hurts." This song is performed by Lee Sang Gon of the four-member male R&B group Noel. It serves as the theme song for the love story between the mythical Gu Wol Ryeong and his human wife Yoon Suh Hwa, played by Lee Yeon Hee. It's the drama's most often played song. None of the main characters in this drama will escape the pain of love.

The third song, "Love is Blowing," is performed by ballad singer Lee Ji Young of the female singing group Big Mama. The tempo is upbeat and urgent, with romantic lyrics that include phrases such as "even the stars become silent when they look at us."

Finally, the most popular song to come from the soundtrack is "Spring Rain," performed by pop and ballad singer Baek Ji Young. This song evokes the love story between the half-human Kang Chi, played by Lee Seung Gi, and the human Yeo Wol, played by Suzy. Themes in this song also reflect back to the heartbreaking story of Kang Chi's parents with lyrical phrases such as "love comes to me like the spring rain and silently melts my frozen heart." Piano and string instruments serve to heighten Baek Ji Young's sorrowful vocals.

It's not the first OST for Baek Ji Young, who has rightly been nicknamed the "Queen of OST." She has contributed her powerful voice to the soundtrack of several dramas, including "Secret Garden," "Rooftop Prince," "Greatest Love" and "Iris."