If the word ends in the sound:

[1] t or d, the past tense suffix -ed is pronounced [Id].
[2] b, v, w, m, n, l, r, z, y, g, the past tense suffix -ed is pronounced [d]. And when the word ends in a vowel sound?
[3] p, f, s, sh, k, x the past tense suffix -ed is pronounced [t].