Thank you both George and Manyakumi~!
Manyakumi, George said you are his "definitive grammar source! "
I do envy you!  
And George, I am  

  at you!  
I thought I am your "definitive grammar source! "
But actually I am not!  
 
 
On the other hand, it's great to see the teamwork two of you perform!
   

   I am more than "happy"!  (but still I am  

  at you, George!)
Your explanation (both of you) is excellent!
In order to explain this grammar point, it's better to put them into example sentences. 
1. 갔을 길 - 이 길은 내가 갔을 길이야.  저것은 내가 갔을 길이야.  
As Manyakumi just mentioned, "the path that would have been through (never been actually)"  아마도 이 길은 내가 갔을 길이야!  "Perhaps, this path would have been my path!"  (never been actually but he thinks he would)
2. 갔던 길 - 이 길은 내가 갔던 길이야.  저것은 내가 갔던 길이야.
As manyakumi mentioned, "the path that have been through"  
이 골목은 내가 어렸을 때 갔던 길이야!  "I used to go by this alley when I was young!" 
Now I do agree with George!  
Manyakumi is my "definitive grammar source" as well!  
Thanks for your thoughtful explanation!  
I really do appreciate both of you, George and Manyakumi!  
best,
Tim   
 
