Ordering of English sound changes in verbal morphology
As we all know, the Early Modern English 3sg verbal ending -eth has become -s in Modern English. This presumably happened in two steps:
- Elision of the unstressed e in the final syllable
- Changing final -th to -s.
But do we know what order these changes happen in? Do we find intermediate forms like speak'th (with elided vowel but no consonant change) or speakes (with consonant change but the vowel preserved)?
In Old English, a final -th without a vowel before was attested in the conjugation of 3p. sg. verbs. For example, the verb, steal was stilð in the 3p. sg.
This also occurred in words such as heal (with hǣlþ) and say (with segð).
In these words, it is easy to see how a final þ and ð could become alveolarized as /s/ and /z/, respectively.
This means that in these words the final unstressed e had already been elided. From this, we know that the order of events, at least in these words, are as you listed: fisrt the elision of the e, and then the change from -th to -s.