@Aliyyah Rusdinar
I already answered this and it is included in the FAQ.
The king has multiple wives/queens, was that really normal back then?
Yes, it was normal, and it was not just done in Korea, many kings from all over the world usually had multiple wives. You might find it weird because in our current time polygamy is not frequently seen. But it was a common practice by nobles back then to further increase their power or create unity between kingdoms.
@Aliyyah Rusdinar
I want to add somethings from what Hessa said. Taejo (the king) is the first king of Goryeo. He built the nation by unified three powerful kingdom in Korea that time (Silla, Baekjae, Goguryeo). As a newborn nation, Goryeo was very unstable, thus, Taejo use the royal marriage as a way to gather powerful families and stabilize the nation. History record Taejo has 29 wives and 34 childs (25 of them were sons). In Goryeo period, unlike Joseon, there isn't one queen consort. All of the queen married to the king is queen. Therefore, Taejo has 29 Queen. But only 2 got portrayed in this drama. Fascinating right? All of the queen has the same status and rank. Thus, every eldest son of the queens could become the crown prince. The only thing matter in this period is the family background. That's why the 3rd look rather superior than the 8th. That was because the 3rd's mother's family is more powerful than the 8th.

Actually, all of this was informed in the special episode of moon lovers (episode 0). But as far as I know, no one subbed this yet.
@Hessa - I noticed an error in the FAQ (under the heading: How to know which version I'm are watching?):

If you are watching in DramaFever or Kissasian, it is the SBS re-edited version. If you are watching on hdfree or dramalove, it is the original version.

It should be:

DramaFever & HDFree = Original International ver.
KissAsian & DramaLove = SBS Re-edited ver.
i still don't get how Princess Hwang Bo Yeon Hwa loves 4th prince? isn't he her brother somehow?
@Yozzora - This is covered in the FAQ (initial post of this thread).

Yes, they're half-siblings (same father, different mothers).  Back in the day it was normal/acceptable to marry blood-relatives: cousins, half-siblings, full-siblings, etc.
Hessa u noted this "If you are watching in DramaFever or Kissasian, it is the SBS re-edited version"

My friend tell me that Dramafever actually HAS the original version and NOT the re-edited SBS version. she watch the show through Dramafever and went back to check the episodes, and the scenes included in the SBS version are not on there.

I didn't watch on DF so I dont know, correct me if Iam wrong. Thx u ^^

@BlueberryCheesecake .... @@Aliyyah Rusdinar  ... I just want to add that Wiki lists 6 of the 29 consorts as "Queens", the rest have the title of "Lady" ... Obviously, Queen has more power than Lady ....  I don't know if they have the system where the first wife, is given dominance over the others ....
maybe b/c he had to build alliances, all 6 queens were given equal power/importance!?!? 

@Neyjour ... @vieedha .... @Hessa ..... DramaFever has the International version, I checked several times  ... however, I saw somewhere that Dramafever also has subs for SBS version.... thus the confusion!

I just learned yesterday that hdFree has SBS Raw episode 9 and International Subbed Ep 9 .... Not sure if it is the same for other episodes.
I got confused watching raw on hdFree than seeing same version on Kissasian...LOL!
@rainruma really? I just know this fact. Yeah maybe all 6 has queen title to give equal power among the powerful house clan. Afterall, Goryeo didn't have systematic rules like Joseon did. Even the decent system of court lady (Gungnyeo and sanggung) started at joseon period. That's why, gwangjeong killed many of his cousins and half brothers (as history said) because he need to stabilize the nation. I remember Queen Yoo said in earlier episode "How did the king hope goryeo would be after his death? Split it in to 29 nations?" and she's not totally wrong haha. The equal power the queen and the house clan has really bring separation and struggle of power. Because everybody equal, everybody thinks they have the right over the kingdom.

Anyway, I read on dramabeans that currently a team has been formed to plead sbs to release dvd blue ray of Moon Lovers. They plan to demand sbs to gather the two versions of the drama as well as the scenes that didn't make the final cuts altogether and release it as dvd
thank you hessa !! inceredible knowledge
@BlueberryCheesecake --- I need to read more about Goryeo :D

That is very interesting about DVD.... I was just thinking today that someone should combine both versions of each episode so that all the missing scenes will be included in one episode.... 
The editors are really bad, even now... In both versions they cut out important scenes, while keeping some non-relevant scenes that seem more like filler.

I really would have liked  to see a K-version focused  on 4th and the romance, but done in a more deep and mature way! ..
Producers  messed up a wonderful opportunity .... They also seem to lack a clear vision as to which path to take.

I have a feeling the famous Korean nepotism is at work here!!! :P:P
Thank you Hessa and everybody else who has commented on this forum!! :) Helped a lot with some doubts that I had!!
Dramafever has the international version but sbs version sub? That is weird okay, I will fix it. Sorry for the mistake. I assumed it was the SBS re-edited version because of the subs.
@Hessa --- DF shows Int'l version subbed accurately.
the SBS subs are somewhere, Not  on DF site !! .... I saw a link 2 weeks ago about downloading SBS-version subs only,  and the subs said DramaFever at the start..... I don't remember where I saw them, sorry.
Yeah, I've seen it mentioned several times before that DramaFever had the SBS Re-edited subs (even though the actual video is the Original International ver.) and thought that was a bit odd.  Because of extra/cut scenes and the same scenes appearing at different timestamps... the subs couldn't match up correctly, right?  Err... I dunno.  Honestly, this whole thing with two different versions is extremely annoying.  So much confusion, and I hate having to jump back and forth between the two to make sure I didn't miss anything.  I really, really hope this doesn't start to become a common practice with dramas (from any country).  :(

@rainruma - Oh, I didn't realize HDFree had RAWS of the SBS Re-edited version (I never watch RAW since I only understand a tiny bit of Korean).  Hmmm... even more confusion.  Oh joy.  :(  But I just checked my files, and all the subbed episodes from them so far (1 - 9) are definitely the Original International ver.
I checked Dramalove and they have both versions. Episode 6 is the SBS version. However, episode 8 is the international version.

I always download the SBS version subs to watch the extra scene but the subs start with "Subbed by Dramafever", so I automatically assumed that Dramafever has the SBS version. Why the hell does those subs say "Subbed by Dramafever" when it doesn't belong to DF!?!

This is so confusing. Why didn't the production company do a good job editing beforehand instead of causing this!? This so annoying.