I can't put it into words.
 →   말로표현할수없어.

[예문]
A:I've got a big problem.What can I do?
  문제가 생겼어. 어떡게하지?

B:What's wrong?
  뭐가 잘못된거야?

A: I can't put it into words.
  말로 표현할수가 없어.