Much of the time when it's at the beginning of a word or phrase or when it follows a consonant the Spanish d
is pronounced almost the same as the "d" in words such as "dog" and "Adam." In Spanish, however, the tongue may be a little bit farther forward, on the gums at the top of the teeth rather than the ridge between the tooth line and the roof of the mouth. This makes the Spanish sound a slight bit less explosive than the English sound, perhaps like the "dd" in "griddle" for some speakers.
When the d comes between vowels or at the end of a word or phrase, its sound is much softer. It is a voiced fricative with the tongue touching the bottom of the top teeth, meaning it sounds much like the "th" in "the" or "bathe," but a bit shorter in duration.
Words and phrases spoken by native speakers in our audio lesson on pronouncing the d are buenos días, dispénseme, adiós, ¿cómo está usted? and de nada.