Listen to yourself read this passage aloud:
I stood naked on the rugged, jagged precipice, and faced my accursed, aged foe with dogged resolve. The wretched, wicked wizard stretched a crooked finger from a ragged sleeve. As he began to mutter the incantation, I thought of my beloved Hildegarde.
How’s that to set you on the edge of your seat? Aside from the gripping drama and suspense, what did you notice? Maybe you noticed that it had way too many adjectives. True enough, but there’s more. Every one of those adjectives—naked, rugged, jagged, accursed, aged, dogged, wretched, wicked, crooked, ragged, and beloved—has the same unusual pronunciation of the suffix –ed. If these words were pronounced like most English words ending in –ed, they’d be pronounced /nekt/, /rʌɡd/, /ʤæɡd/, /əkɝst/, /eʤd/, /dɑɡd/, /wɪkt/, /rɛʧt/, /krʊkt/, /ræɡd/, and /bɪlʌvd/. That’s because usually, the –ed suffix is just pronounced as a d or t at the end of the last syllable of the word it gets suffixed to. It’s pronounced as a d when the base word ends in a vowel or a voiced consonant; for example, agreed, grabbed, hummed, raved, writhed, sailed, roared, leaned, buzzed, bridged, hugged, and longed. It’s pronounced as a t when the base word ends in a voiceless consonant; for example, flapped, puffed, toothed, hissed, scratched, and looked.
Sometimes the –ed suffix is pronounced as its own syllable. This happens when the base word already ends in a d or t sound; for example, floated and braided. But none of the adjectives we’re talking about have a d or t sound before the –ed suffix, so it’s strange that their –ed suffix should be pronounced as a separate syllable.
Instead, they have a mixture of consonants before the -ed. Most of them have a k sound there (e.g., naked, wicked, and crooked), or they have a hard g sound before the -ed (e.g., ragged, rugged, jagged, and dogged). A couple of them have a ch or soft g sound before the –ed: wretched and aged. One more has an s there: accursed. Lastly, there’s beloved, whose base ends with a v sound.
Furthermore, I didn’t even include all the adjectives with this pronunciation quirk in my sentences. I left out learned and blessed, and still others whose pronunciations vary depending on dialect: /fɔrkəd/vs. /fɔrkt/ [forked], /pikəd/ vs. /pikt/ [peaked], /əlɛʤəd/ vs. /əlɛʤd/ [alleged], /səpozəd/ vs. /səpozd/ [supposed], /lɛgəd/ vs. /lɛgd/ [legged], and /strɑɪpəd/ vs. /strɑɪpt/ [striped]. And those are just the ones that I knew about.
I was surprised to learn that as late as 1839, the words booked, tusked, tressed, scabbed, crabbed, chubbed, stubbed, shagged, snagged, scrubbed, scragged, hawked, and stiff-necked were also pronounced with a separate syllable for –ed, at least according to a book published that year called A Critical Pronouncing Dictionary and Expositor of the English Language, by one John Walker.
In short, there’s nothing that these consonants have in common that separates them from other consonants.
So is there some kind of rule for when we pronounce the –ed suffix as its own syllable? There are a few observations that can be made, but it’s not going to be definitive.