D is an Important Letter
This is one of the first things that should be taught for Spanish pronunciation. In my opinion, it is one of the most obvious evidences of a non-native speaker, and it is so easy to fix.
The Difference between English and Spanish
The D in English is very much fully in the mouth and the tongue actually doesn’t do much work. Is the completely the opposite in Spanish. The tongue is in full motion for this letter and is comparable to an English “th.” With that in mind, the tongue protrudes from the mouth slightly less with the Spanish D than with the English th.
How to Perfect a Spanish D
If you are familiar with an American “th” it is essencially the same concept. The tongue protrudes ever so slightly from the mouth, with the tongue touching the bottom of your top teeth. It should look like you are about to bite your tongue, except your teeth aren’t pressing on your tongue. Your tongue is pressing on your top teeth. As you make the D sound, move your tongue slightly downward, and even more slightly inward.
Practice Makes Perfect
Practice in a mirror to ensure you are doing it correctly. Practice with the D in varying parts of different words: dia (day), idea (idea), mitad (half).