The pronunciation of the -ed in regular verbs in the past

The -ed ending in regular verbs has 3 possible pronunciations: /Id/, /t/, or /d/.

When the last sound of the verb is /t/ or /d/, the -ed ending is pronounced as /Id/, as in hated /reited/, and needed /níded/.

When the last sound of the verb is unvoiced, when you don't need to use your vocal chords to produce it, the -ed ending is pronounced as /t/, as in finished /finisht/, and worked /uorkt/.

When the last sound of the verb is voiced, when you have to use your vocal chords to produce it, the -ed ending is pronounced as /d/, as in cleaned /clínd/, and played /pleid/.

If you'd like to check some more things about this topic, you can take a look at http://www.bbc.co.uk/worldservice/learningenglish/radio/specials/1413_gramchallenge26/

Enjoy!