How to naturally evolve verbs into adverbs or dependent clauses?

Question

Currently I have a very simple proto-language which I'm trying to evolve. For example:

slishi  hu      ho    shofli  shofli pfufi   tushi
repair  person  tool  fish.V  fish.V succeed cause
Person repairs fish net to have abundant catch.

However, this seems to be confusing and unwieldy, so I'm assuming that native speakers would feel the same and would attempt:

• To change tushi into something similar to 'so that' combining two sentences where one causes another.
• To change pfufi into an adverb - successfully

Something like (for example):

slishi hu      ho    shofli tushu   shofli  nshi     pfufe         he
repair person  tool  fish.V so.that fish.V  AUX.FUT  successfully  3.PRO
Person repairs fish net so that he will have an aboundant catch

Note that tushi (to cause) has been changed into tushu (so that) to make a conjunction and pfufi (to succeed) into pfufe (successfully) to make an adverb.

However I'm not sure what a natural way of doing this would be. Most guides for conlangs suggest to apply phonological evolution rules universally, so I'm not sure if it is natural to diverge [-i] into [-ɨ/e] and then [-u/e] to derive an adverb/preposition/conjunction and suggest to evolve the adverbs from nouns or verbs, but they don't provide guides for creating a derivation systems (that I found).

If this helps, the language later/earlier evolves tenses by adding the pronouns - ignoring other changes that happened in between:

slishi-he  hu     ho   shofli  tushu   shofli nshi    pfufe         he
repair-1   person tool fish.V  so.that fish.V AUX.FUT successfully  3.PRO
Person repairs fish net so that he will have an aboundant catch

Answer 1

This is more a lengthy comment on your question than a real answer, but comment space is restricted ...

You are undertaking a very difficult task by setting a starting point and an end point and asking for natural development between the two. This is difficult even when it is known that such a natural development exists because the two languages are attested natural languages and not constructed.

We know sound laws really well, and also the process of grammaticalization is fairly well understood. The evolution of different basic word orders is still under research, we know that it has happened but how exactly it happened, is less clear.

It seems that your are needing some grammaticalisation to get from the verb pfufi to the adverb pfufe. Here's a suggestion: Add a particle ye meaning roughly "like" to your proto-language, and the phrase pfufi ye will fill the role of an adverb. It is easy to conceive, that pfufi ye evolves to pfufe by just dropping one syllable (syncope). To make tushu from tushi a particle like vu might be applicable, but I have some difficulties to define its original meaning.

I think that language change is easier to "grow" than to "plot", i.e., it is easier to just apply changes to a proto-language and watch where they lead to than to interpolate between two stages of a language.

Of course you can invoke language contact to achieve a particular change in your language evolution, as kind of last resort to save the plot. Just don't overuse this.