I thought about posting this in the conlang forum, but that forum seems to be pretty much dead. Besides, this question is more about coming up with rules for sound change than anything else.
Mark Rosenfelder claims somewhere (I forgot where, I can't seem to find it in my books) that you can simply derive inflections from agglutinating endings, which is how they come about in natural languages. But thinking about it, I can't imagine what forms they may take.
For example, let's say you had a conlang that had inflections for tense, person, and number. That would mean there's three sets of suffixes, one for tense, another for person, and another for number. Just for the sake of discussing this, I'll throw together some half-assed suffixes for us to play around with
future: -pa
present: -Ø
past:-ma
1p: -be
2p: -de
3p: -se
plural: ne
So, just for an example, that means that the first person plural past would be derived from ma-be-ne.
But how would this be reduced to one or two syllables? I'm aware of things like epenthesis, elision, lenition, and such, but how do you decide what gets removed and what phoneme becomes what?
Really the only two sound changes I've heard of are t and s becoming palatalized before an i or some other front vowel. And I've heard of laryngeal consonants turning e's into a's in PIE. Oh, and inter-vocalic consonants becoming voiced. But other than that, I'm clueless.
Is there any actual evidence of agglutinating endings turning into inflections or is that just speculation like Mark Rosenfelder's theory on the semitic languages developed its odd tri-lateral root system? Is there any actual examples people can show me?
edit: Oh, and I am aware of the origin of stem changes: they've simply a consequence of vowel harmony. U moves to the front and becomes y, which then becomes un-rounded and thus turns into i (this is how the plural of 'goose' became 'geese'). Also, the stem-changing past tenses of English (speak-spoke, take-took, bring-brought, etc...) came from the vowel stem harmonizing with an old past-tense marker, which was then dropped since the stem-change made it redundant (this is also why we say 'geese' rather than 'geeses').