How to pronounce the final -ed in verbs

The final -ed in verbs in the past or past participles has three different pronunciations, depending on the sound that precedes it.
Those verbs ending in /d/ or /t/ need to have another syllable so that people realise that they are in the past.
decide /dɪ'saɪd/ --> decided /dɪ'saɪdɪd/
want /wɔ:nt / --> wanted /wɔ:ntɪd /
In the above examples -ed is pronounced /ɪd/.
In the rest of the verbs, -ed is pronounced either /t/ or /d/, but how do we know which? It depends on the final sound of the verb: if it is voiced (sounds in which the vocal chords vibrate), the suffix is pronounced /d/, because it is a voiced sound as well. If the consonant is unvoiced (no vibration of the vocal chords), it is pronounced /t/, as it is also an unvoiced consonant. As you can see, it is very simple: the suffix takes the sound that is easier to pronounce in each case.
Voiced consonants are: /b, v, ð, d, z, ʒ, dʒ, g/
Unvoiced consonants are: /p, f, θ, t, s, ʃ, tʃ, k/
Put your hand on your throat as you say the consonants above and you will feel the difference.

Try doing this exercise and check how much you have learned.