1. The /id/ sound
If the last letter of the word is spelled with D or T, the ED is pronounced as a separate syllable with an /id/ sound (it rhymes with kid and lid).
wanted (sounds like "want-id")
waited
needed
folded
2. The /t/ sound
If the last consonant of the word is voiceless, then the ED is pronounced as a T. Be careful not to create an extra syllable or "id" sound.
talked (sounds like "talkt")
kissed (the S sound comes from the front of mouth so it would sound like "kisst")
parked
helped
NOTE: As an example, with the word "helped", if you say "help-id" as a two syllable word, then it is very likely that people will NOT understand the word or what you are saying. (It is pronounced like "helpt")
3. The /d/ sound
If the last letter of the words ends in a voiced consonant (or sound), then the ED is pronounced like a D (without creating another syllable)
played (sounds like "playd")
closed (the S sounds like a vibrating Z so the word would sound like "clozd")
opened
lived