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.
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