American English Phonetics

2.2 The past (participle) endings

The English plural morpheme has three shapes (or ‘allomorphs’): /ɪd/, /t/ and /d/. They are used as follows:
 

Shape 1

/ɪd/ after /t,d/, as in fitted, loaded;

Shape 2

/t/ after the fortis obstruents /p, k, f, θ, s, ʃ/, as in helped, knocked, stuffed, pronounced, leashed;

Shape 3

/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:

marked	/mɑrkt/	markedly	/ˈmɑrkədli/
assured	/əˈʃʊrd/	assuredly	/əˈʃʊrədli/

but: good-humouredly /ˈgʊd ˈhjuːmərdli/.