MATCHING BEHAVIOUR IN ENGLISH SUFFIXES

Four suffixes in English - the plural -s, the possessive -'s, the third-person singular -s and the past-tense -ed - behave in terms of pronunciation in ways that are weirdly parallel. Like the good linguists we are, we'll start with the data and derive the rules (or rather the patterns) from it:

Plural suffix:
row ɹəʊrows ɹəʊz
robe ɹəʊbrobes ɹəʊbz
rose ɹəʊzroses ˈɹəʊzɪz
rope ɹəʊpropes ɹəʊps

Possessive suffix:
Joe dʒəʊJoe's dʒəʊz
Job dʒəʊbJob's dʒəʊbz
Roach ɹəʊtʃRoach's ˈɹəʊtʃɪz
Hope həʊpHope's həʊps

3rd-singular suffix:
boo buːboos buːz
jog dʒɒgjogs dʒɒgz
push pʊʃpushes ˈpʊʃɪz
look lʊklooks lʊks

Past-tense suffix:
boo buːbooed buːd
jog dʒɒgjogged dʒɒgd
pat pætpatted ˈpætɪd
look lʊklooked lʊkt

Let's take z to be the normal ('unmarked') pronunciation of the three suffixes with -s. They all behave in the same way:
- after a vowel, the suffix is voiced, z (ɹəʊz dʒəʊz buːz).
- after a voiced consonant, it's still z (ɹəʊbz dʒəʊbz dʒɒgz).
- after a consonant of the same family as the suffix (in this case a 'sibilant'), the suffix has a vowel inserted before it, creating an extra syllable. The 'sibilants' in English are s z ʃ ʒ tʃ dʒ (ˈɹəʊzɪz ˈɹəʊtʃɪz ˈpʊʃɪz).
- after a voiceless consonant, the suffix devoices to s (ɹəʊps həʊps lʊks).

(There's one caveat to add to this description: many speakers resist adding the extra syllable when adding the possessive suffix to a noun ending in a sibilant, saying James' mobile ˈdʒɛɪmz ˈməʊbaɪl instead of James's mobile ˈdʒɛɪmzɪz ˈməʊbaɪl. Purists and pedants, however, add the extra syllable, and that is also Derek Rogers's ˈɹɒdʒəzɪz preferred form. That's true for the plural of names as well: we're the Rogerses ˈɹɒdʒəzɪz, not the Rogers.)

The past-tense suffix follows the same system:
- after a vowel, the suffix is voiced, d (buːd).
- after a voiced consonant, it's still d (dʒɒgd).
- after a consonant of the same family as the suffix (in this case an alveolar plosive), the suffix has a vowel inserted before it, creating an extra syllable. The alveolar plosives in English are t d (ˈpætɪd).
- after a voiceless consonant, the suffix devoices to t (lʊkt).

Let's just bow down in amazement that they all work in the same way.