'Soft' d versus 'hard' d.
When do you pronounce a 'd' hard (touching the roof of mouth), and when do you pronounce it soft (the kind of soft 'th' sound)?
When pronouncing a hard d, the tip of the tongue should touch the back of your teeth, not the roof of your mouth.
I got scolded the other day for a soft "d" when saying "todos", the pronunciation must be a clear "d" There are very few "hard" sounds in spanish,but even considering that, the pronunciation must be clear.
We usually think of hardness of pronunciation of the D as the amount of TH sound put on it. No D in Spanish is pronounced with the hardness of the English D. It´s a matter of soft and softer.
¿Donde está? (1st D less soft than the 2nd).
Está en la ciudad. (middle D softer than D at the beginning of a word. The last D is the softest - greatest amount of TH sound).
I agree with Lorenzo. Regardless of the intensity of the D, the tongue touches the back of the front teeth.