Help for Korean pronunciation problems

The single sound is the sound of D. But in a word, if the pronunciation of the previous word is the pronunciation of the tip of the tongue against the roof of the mouth. The D at the back will naturally sound like a T. This is not a wrong pronunciation, but a process of evolution in natural pronunciation. If you use the D sound, you have to pronounce it a bit awkwardly. The sound produced also feels stiff.

You will find this pattern if you read more words. You will find out if you try it carefully.

Hope it helps.