Translation:Twelve thousand three hundred ten
만 means ten thousand so there is only one ten thousand in this. The 이 is modifying the 천. So in Kevin's post you see that 10,000 is alone first (만) Then add 2 (이) time 1,000 (천).
If it said 이만 then it would be twenty thousand. There would be two ten thousands.
Using won in Korea helped me because there is a 만 원 bill. If you printed out some fake korean bills and pretended to buy things or tell someone how much something is it might help.
yes, its considered incorrect, im gonna use french as an example.
the word for 90 in french is 4+20+10, because 4x20+10=90 (which is wack but anyways)
there is no standalone word for 80 in french, its just that^.
think about @absxyz0123 example as the opposite. you have to use the larger unit in korean because people won't understand what you're trying to say if you throw numbers in a random order like french does