If the word ends in
 t or d, the past tense suffix -ed is pronounced [Id].
 b, v, w, m, n, l, r, z, y, g, the past tense suffix -ed is pronounced [d].
 p, f, s, sh, k, x the past tense suffix -ed is pronounced [t].
Note,  share the same place of articluation with -d- of -ed, so an extra vowel, [I], is inserted to ease articulation; are voiced sounds, so the past tense suffix [d] is voiced;  are voiceless sounds, so the past tense suffix [t] is voiceless.
The rule: /-ed/ changes pronounciation according to its environment. If it follows a voiced sound, it too is voiced, [d]; if it follows a voiceless sound, it too is voiceless, [t], and if it follows the same sound, then a vowel is inserted to ease pronunication, [Id].
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