All of these share the f-l root. But how did this turn into the Spanish hallar? Well, first remember that the initial F- sound tended to disappear when Latin turned into Spanish; see, fig and higo or fable and hablar. Secondly, note that finding something is just blowing on it, uncovering what was below the dust you blew away!
From the same root, however, also comes the English ineffable, that SAT word meaning “unable to be described in words.” So, ineffable literally means “without” (in-) and “speaking” (fabulare).
We see the h-b-l of hablar map to the (in-)f-b-l of ineffable quite clearly!
Fervor is really just an intense passion heating up. Thus we shouldn’t be surprised that it comes from the Latin root fervere (“to boil”), from which we get the Spanish for the same (“to boil”), hervir.
Thus, the f-r-v of fervor maps to the h-r-v of hervir.
The Spanish for “son”, hijo, doesn’t sound like anything in English. But it is a close cousin of the English synonym for brotherliness: filial.
Both come from the Latin for “son,” filius. The transformation to Spanish came about through two interesting patterns: the initial f- in Latin usually turned into an h- in Spanish (such as, hacer and fact, or hablar and fable). The other pattern is less common: the -li- sound turned into a -j- sound – it’s just a less common sound! Thus the f-li maps to h-j almost exactly.
From the Latin filius, we get a few other English words, including: affiliate: an affiliate is, in a way, a child you rear!
From the same root we also get the English fetus, fecund and even feminine. These come, via the Latin filius, from the Proto-Indo-European root *dʰeh₁y-li-os, meaning, “sucker” — in the literal sense of, “one who sucks.” Children, indeed, are defined by their sucking their mothers; so your hijo is literally, “the one who sucks.” And, some might argue, even affiliates themselves usually do suck!
“Fig” comes from the Latin “Ficus” — obvious enough!
But, curiously, the Spanish word is “Higado”. Huh?