I was taught that Spanish d has two pronunciations: at the beginning of the word it is a hard d; between vowels, it is pronounced /ð/, much like th in English the.
Please look at this sentence:
Nosotros nos reunimos para dar la bienvenida a la profesora.
Should I say /para ðar la/ or /para dar la/?
This is a good question, and unfortunately the answer is, "it depends". The Spanish letter d has different pronunciations depending on where it comes in a word.
Word-initially, it will generally have a sound closer to the English d, although pronounced with the tongue behind the teeth, rather than upon the upper alveolar ridge (on the hard palate).
Intervocalically (between vowels), the Spanish d is normally realized as /ð/, that is, the same sound you make when you say the word rather. In some dialects, it may be /Ø/ (elided/silent) if it comes before the last syllable.
Word-finally, the pronunciation will vary a bit more. The four most common pronunciations are /d/ (like word-initial), /t/ (also with tongue behind teeth), /θ/ (as in thistle), or /Ø/. This is heavily dependent on dialect.
Now you'd think the answer would be easy. dar has a word-initial d, so it should sound /d/. However, in Spanish, many words will often be pronounced together as if they were a single word, especially in fast speech. Prepositions, object pronouns, demonstratives, and articles are the most likely to be "joined" to a more significant word, generally a verb/noun/adjective.
I would normally have the following word groupings in speech:
Why is this significant? Because each group is pronounced as if a single word, that is, as if it read nosotros nosreunimos paradar labienvenida alaprofesora. In this case, you can see that the d is now positioned between two vowels, and would thus be pronounced /ð/.
On the other hand, let's say you trail off after saying para because you were deciding between saying dar la bienvenida and saludar. Notice how the grouping changes:
Now dar is back to its own word group, and will get the word-initial pronunciation of /d/.
Actually, we Spanish speakers are not aware that 'd' has different sounds.
There might be different sounds (I'd say it depends on regions) but to me (Argentine) our 'd' is similar to English 'd', only that a little softer.
You can pronounce that way always and it won't never sound wrong. We never stick the tongue between-out of the teeth (as in "this").
I agree with Leonbloy. While I found that in some cities of Spain the "d" at the end of the words sounds like "z":
...in Argentina, I never heard someone pronouncing the "d" any different being at the beginning, the modle or the end of the word.
For your reference, in Spanish each vocal preceded by a D will sound:
da: as in DAd
de: as in DElaware
di: as in CanaDIans
do: as in DOnald
du: as in DUbai duːˈbaɪ (like doo-BYE) -could not think any better example!
Nosotros nos reunimos para dar la bienvenida a la profesora. You could also omit thenosotros.