Pronounciation of the word the

Hey I am asian, and I have been brought up to pronounce any th consonant as ''d.'' But recently, I have learned to correct my habit and say that, instead of dat, them instead of dem...you get the story. However, there is one word I can't seem to hear the difference, that word is the. I can't hear if it's THuh or Duh. I can hear it if it's the say one word. But when I talk in a conversation, I can't hear it. Duh cat ran over Duh fence to go to Duh basketball court. Thuh cat ran over Thuh fence to go to Thuh basketball court. I can't hear it when spoken fast. How can I train my ear to hear it, there is very slight distinction when you speak it in sentence compared to when you say it one word at a time.