What's the difference between 길이 and 길이?

ANSWER: One means length, and the other means for a long time. In other words, when 길이 is used as a noun, it means length, and when used as an adverb, it means for a long time.

Today I came across the expression 중국사에 길이 남을 쾌사(快事), which can translate as a joyful event (쾌사) in Chinese history that will last the ages. Here 길이 is being used as an adverb, so it means for a long time or forever, and 남다 means to remain, to be left over, or to survive and, therefore, can be translated as to last. Here are a couple more examples from my dictionary of 길이 being used as an adverb:

• 그의 이름은 길이 남을 것이다.
  His name will be immortal [will live forever].
   
• 그분의 은혜를 길이 못 잊겠다.
  I shall never forget his kindness.

And here are some examples of 길이 being used as a noun meaning length or extent:

• 길이 일곱 자의 뱀
  a snake with a length of seven feet
   
• 각각 1미터 길이로 잘라주시오.
  Cut them in lengths of one meter each.
   
• 길이는 얼마냐?
  How long is it?

In my dictionary, both the adverb 길이 and the noun 길이 are listed as very frequently used words, which means they should be at the top of a list of Korean words to learn. And 길이, of course, should be pronounced as /기리/.