Although the ending was a little confusing, it was still a good film. I'll try to explain the finish to those who don't comprehend it.
When Daeun met Lee Jang Hyeok (the engineering guy), she was in a relationship with Jang Hyeok (Jang Ki Yong) (Lee Woo Je). Lee Jang Hyeok, who was hospitalised with Hepatitis B, spent a long period in the hospital and afterwards developed a crush on Daeun.
Daeun, whose relationship with Jang Hyeok is already crumbling. Lee Jang Hyeok realises she isn't wearing her ring because she has already given it back to him (the scene where Jang Ki Yong received a delivery at the office)
When Lee Jang Hyeok (Lee Woo Je) confesses to Daeun and the two spend the night at her residence, Lee Jang Hyeok (Lee Woo Je) later changes the bulb light for her.
I'm guessing she's already dumped Jang Hyeok (Jang Ki Yong). Remember how the Macaron Man said "you come again" at the start of the movie? That was the night she was dating Jang Hyeok, and they fought in the car.
Daeun invited Lee Jang Hyeok (Lee Woo Je) to go to Jeju using the ticket she had already booked with her ex-boyfriend at the airport (Jang Ki Yong). When Jang Hyeok went to the airport to apologise to Daeun, he ran into Lee Jang Hyeok, who was sprinting and showing off the sneaker she got for him. Later, Jang Hyeok realises that it is too late, and Daeun is now in a relationship with Lee Jang Hyeok.