How to trace Proto-language roots towards English and French?

Question

TL;DR: What resources tie Proto-language roots (eg Proto-Indo-European), to English and French, especially if spelling has changed? I always heed linguistic pitfalls, but I always try to find some right ways of interpreting this language, so that it feels reasonable and intuitive.

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For example, I'm suffering this difficulty with the noun 'faith'. OED doesn't track far back enough. Etymonline is too terse; it doesn't explain how the spelling of fidere evolved from * bheidh-:

from Latin fides "trust, faith, confidence, reliance, credence, belief," from root of fidere "to trust," from PIE root * bheidh- "to trust"

Then I can try to learn words top-down and from the top of the tree model, where I can oversee, behold, and absorb all the fascinating links and networks between seemingly unrelated words

Answer 1

The first thing to do is stop thinking about spelling, and start thinking about pronunciation. Proto-Indo-European had no spelling, and for a long time, neither did Latin.

In this case, you need to understand the sound changes from PIE to Latin (for a word like "hundred", you would need to know the sound changes from PIE to proto-Germanic). Two useful books are Buck's Comparative Grammar of Greek and Latin and Brugmann's Elements of the Comparative Grammar of the Indo-Germanic Languages. The latter is more complete and will include Germanic, not to mention Celtic, Indic and so on (leaving out the later-discovered languages), but Buck might be easier to use / understand.

I found it most useful to pay attention to English word pairs that come via Latin vs. Germanic, like "abide" (Germanic) and "faith" (Latin), which made it easier to remember concrete examples and thus the various sound laws of Latin and Germanic. Though the list is short enough that you can just memorize the main rules. Consulting the online Walde-Pokorny is also helpful. (Amazing! you don't even have to go to the library anymore).