The English plural morpheme has three shapes (or ‘allomorphs’): /ɪd/, /t/ and /d/. They are used as follows:
/ɪd/ after /t,d/, as in fitted, loaded;
/t/ after the fortis obstruents /p, k, f, θ, s, ʃ/, as in helped, knocked, stuffed, pronounced, leashed;
/d/ in other situations, i.e. after /b, g, v, ð, z, ʒ, m, n, ŋ, l, r/ and after all vowels, as in stabbed, bagged and in showed, tied, etc.
In irregular verbs these rules of course do not apply.
Note that –edly is always pronounced /ədli/ when the accent falls on the last syllable of adverbs:
but: good-humouredly /ˈgʊd ˈhjuːmərdli/.