Let's give each of our opinions on these mysteries/confusing things. 

Questions you have and I have. Comment session is not very useful to find answer. But fun. 

Edit:

Let's solve  guess these mysteries so we can get insight from each other. LOL. XD

These are all guess/theory and we will know that they are valid or not when the story finally end.

EP 11 & 12

I see that most are confuse with lighting scar and LG tears when time stopped.
I will explain from how I understanding. I could be wrong too.
Koo Seo Ryung went to Korea and Jeong Tae-eul went to Corea at the same time after JTE collapsed. LG started counting time which means that time stopped. Before that Luna went to Korea when LG threw the King's ring away. Lee Lim stayed those whole time in Corea. Then, KSR went back to Corea and Lee Lim went to Korea to have a meal. xd
1. Luna went to Korea (when throwing King's ring)
2. KSR & JTE swap (when LG counting time in his room after JTE collapsed from poison)
3. KSR went back to Corea and LL to Korea
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np to Scar
JTE & Luna, Jo Eun Sup & Jo Yeong, Noh Ok Nam, Shin Jae    
If people from both world still exist or die naturally, no scar. KSR might have probably killed Koo Eun-a from Korea. She is just too greedy and what she has for LG is not love but greed too. Maybe, murder cause people by difference world will be curse with lighting scar. LL killed young LG in Korea. Each of their counter part is killed so they got cursed.
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LG tear when time stopped in ep 12 because he was sad that she or he would forget each other. That's base on what Noh Ok Nam was reading the poem in the background. He feared that the door to her would close? Time stop forever? He would be too lonely and sad.
Only he and LL still alive? LOL that would be funny. But that's not gonna happen or it would be terrible.

Anyone confuse about the scar on Lee Gon neck?
If you look carefully again, the scar is on the right side of the neck is correct. Director doesn't make a mistake about that. Lee Lim held one half of flute in his left hand and another half in his right hand when the flute got cut in half by Lee Gon. Then, he strangled with his right hand and stab LG neck with the flute on his left hand. He only held half of flute on his right hand. Its was not used to stab. The blood on the left side of LG's neck is from LL's hand. The blood on the right side of the neck was from LG.
I got confused at first too and wonder why director made such an obvious mistake. Haha. I was wrong. The scar on the right side of LG's neck is correct. The scar goes from mid to the right side.

@Baek In Ho 

Four episodes are definitely not enough to develop the growing number of side characters lol when are they going to explain Lee Rim story? I still don’t understand his true motivation 

i thought i am the only one who's still not getting Lee Lim's motive. things they still need to explain:

1. Shin Jae's story

2. Jo Yeong-Eun Sup swap/love stories

3. Lee fckin Lim's motive

4. the exponential increase of time stop - what does it mean for the story?

5. CCTV showing year 2022.

6. first opening scene in ep. 1 showing the interrogation of Lee Lim by Shin Jae and Tae Eul in the police station.

7. who's the one who saved young Lee Gon, and why that person has Tae Eul's ID?

8. Is Court Lady Noh originating from Korea an important part of the story?

9. PM's motives, actions and her doppelganger. did she travel to the other world by shaking hands with Lee Lim?

10. the space of the vacuum of no wind, sun and air where a day there is equal to 2 months in the outside world.

11. LUNA.

12. that scene when Tae Eul sees the jacket of Lee Gon in his chamber which he wears when he crosses over just to give her the blue flower and suddenly left. has that scene not happened yet or what?

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@Pranksalot

You’re definitely not alone. It’s bothering me so much that Lee Rim is barely featured in each episode  although he’s the big bad wolf of the story. 

12. What’s yo-yo boy purpose and story? Is he just a random cheat code for the good guys?

13. Is there a connection between Luna and TE mom having cancer?

14. What does the lightening scars represent?

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15 in total

They pointed good one. Thanks to them we can see the mysteries better.

1. Shin Jae was brought from the Kingdom of Corea, because his doppelganger was dying, and his parents were very wealthy. They were useful for Lee Rim purpose. Maybe  his father was one of the traitors?                                         2. Simple, they will go back where they belongs. Their beloved one are in their worlds.                                                               3. Motive - he should be the King, because he is the oldest one, but he is an illegitimate child. Very popular motive especially in historical dramas.                                                                 4. The time will stop in both worlds and they won't be existed. They have to restore the balance to stop it.                   5. There is a hope that they make it and time will flow again?                                                                                                                       6. This is definitely happen in the Kingdom of Corea - a stamp of connected countries in documents. The near future, because the scene takes place in 2020 or an alternative future which happend in different timeline.           7. It is possible that it is Lee Gon. He travels in time to save Tea Eul. That is why he leaves her pass.                                               8. Lady Noh was the lover of Lee Gon's grandfather, he brought her from Korea. She also may be the Lee Rim's mother.                                                                                                                    9. She probably visited Korea, where she killed her doppelganger. Her motive is power. She wants to become a queen so that people could respect her and admire her. She wants change her social status.                                                                10. Passage to the other world, a place where you can stay and do not aging?                                                                                              11. Tae Eul's doppelganger. She wants to spend her last days in Korea. For me she is not a bad person but very unhappy. It seems to me that Tae Eul will pretend her so that she could recover the second half of the flute. But unfortunately she will die.                                                                           12.   It already happened in Tae Eul's reality, but didn't have a place in Lee Gon reality. Its him from his future.                         13. Not sure. Maybe a balance guardian?                                           14. There is a possibility that Tae Eul's mother is the real mother of Luna. So, she could be from Kingdom if Corea. Or maybe its just random.                                                                                   15. Appears on the people who killed their doppelganger or on those who no longer have him in the other world because he/ she died. I'm not the type of scientist but I think it has to do something with the matter. Scars are a sign of her disturbance. When a given object "human" dies in a one world, its energy disappears. It is possible that this causes some disturbance if a person moves between both worlds, and doesn't have her/his counterpart.

epi 12

how/when did na ri saw her  and eun sup  duplicate  she can't until or unless  she not cross one to another world or they cross

why put the scene " where FL say gun have 2 bullets and ML take gun and fire rest the bullet  is that scene required have some meaning of it "

how yeong recognise prime minister in republic of korea  did i miss some clue

why he wear navy uniform and cooking as mentioned earlier in drama king wear it only in occasion of war  

note: their are many confusion from epi 1 to ------

  i thing  TKEM team has all the ingredient  to make perfect show but the director writter editir  serving it in rush so things are messy 

Na Ri saw her counterpart in a different world because it happens when someone passes between the worlds. And exactly on that time her counterpart was in the hospital. It has happened several times to differen charakters.                      About the gun scene. I not sure it has any meaning. Maybe Tae Eul was in a shock and Lee Gon fire rest of the bullet to calm her down.                                                                                                   Of course Yeong recognized the prime minister because he knows her very well. You can't fool him because he's perceptive, it's his job.                                                                                    There was a conversation between Tae Eul and Lee Gon. She asked him if he only puts on his navy uniform only during the war, and he told her that he would wear it when he will be propose. And it will be take a place on the day when he will be cooking for her. It was a humorous conversation between them but when Lee Gon actually put it on Tae Eul was confused. And she just pretended that she didn't understand the situation.

Can someone explain how lee gon knows that tae eul has been brought to Corea? Because in EP 12 it is shown that he starts searching for her even before she calls the palace... He kept on saying just to wait and he'll come...

He didn't know that she is in Corea. He wanted to find Luna. Detective Kang was looking for her too. When he found her he called to Lee Gon. But on the recording was not Luna but Tae Eul (Lee Gon recognized her).

 Izyda:

He didn't know that she is in Corea. He wanted to find Luna. Detective Kang was looking for her too. When he found her he called to Lee Gon. But on the recording was not Luna but Tae Eul (Lee Gon recognized her).

Oh!!! I get it now. Thanks for the explanation!

EP 12 about Noh Ok Nam/grandma
Noh Ok Nam/grandma probably went to Corea when she was 17 years old because war broke out at 1950. She was born on 1932 so +17 would be 1949. After 1 year in Corea/Korea, war broke out. So, her parents and sister died naturally or by war. Her doppelganger could be in Corea and died naturally just like Luna (She is sure to die even without going to Korea) and young Shin Jae in Korea. So, it doesn't break any gate/universe rule.
LG's grandfather could be the one who took grandma from Korea. I don't think LG's grandfather know about grandma's doppelganger in Corea just like Luna. He might find about her too just out of curiosity just like Lee Gon to Luna. Grandma part is added to explain these I guess.
Therefore, lighting scar is people who steal/murder their doppelganger. This is all just theory. Have to wait for the show's explanation to see it is valid or not.

 This is from YouTube (there is lots of them under the every ost) very interesting theory. "The yoyo boy already hinted Lee Lim. He  the upperhand to make things right by interlinking the fate of two lovers. (Red string of fate). He is the same kid who made the ID card fell into the ditch. And he knows Luna, and Luna knows him (somehow). A guardian or deity of some kind that hates spilling of blood. He's deliberately making stealth moves as a guardian. The ID card accident and CCTV incident."                                                                                  From me I can add here - Luna said that the redthread (motive of destiny, true love) should be replaced, because in the state it is in it will definitely break. An ominous sign about the inevitable death of one of the lovers. For me, this is the death of Tae Eul.

 Izyda:
(Red string of fate). He is the same kid who made the ID card fell into the ditch. And he knows Luna, and Luna knows him (somehow)

Nice one. I thought I have seen that kid. Now, I know. Thanks

When I watched ep 11, I thought that yoyo is a two worlds connecting to red string. 

Two disks = Two worlds

Connector between two disks = Gate

Red string of fate = LG from Corea + JTE from Korea

Anyway, I hope it is not a sad ending. LG & JTE should last forever because of "Eternal" in the Title.

The conversation between the boy and Luna is very interesting, every word matters:                                                             L: All I ask for in return is that you feed Luna.                                 B: Are you stealing something again?                                                   L: Do you believe God exists?                                                                    B: Nodding, yes.                                                                                                  L: Then I'll be punished... By the way that yo-yo...Why is the string red?                                                                                                              B: I Hook it up myself.                                                                                    L: Who tought you that expression? Hook it up again. It looks like it might break?                                                                        Now, I'm not sure if Lee Gon really saved himself. And what if the boy can take other forms? Their meeting was planned from the very beginning. Nothing happens accidentally.

About Lee Lim's motive, in episode 9, Lee Gon figured Lee Lim was after a near-immortal life - eternity, that is why he wants the Manpasikjeok.